There seems at least also an bug in error handling. Disks are gone till a complete reboot.
After a reboot all seems to be fine again ... for some hours.

[138662.040115] ata8: EH in SWNCQ mode,QC:qc_active 0xF sactive 0xF
[138662.040920] ata8: SWNCQ:qc_active 0x1 defer_bits 0xE last_issue_tag 0x0
[138662.040921]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138662.042476] ata8: ATA_REG 0x40 ERR_REG 0x0
[138662.043268] ata8: tag : dhfis dmafis sdbfis sacitve
[138662.044066] ata8: tag 0x0: 0 0 0 1  
[138662.044848] ata8.00: exception Emask 0x0 SAct 0xf SErr 0x0 action 0x6 frozen
[138662.045663] ata8.00: failed command: READ FPDMA QUEUED
[138662.046484] ata8.00: cmd 60/80:00:80:1d:d1/00:00:4a:00:00/40 tag 0 ncq 65536 in
[138662.046485]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138662.048173] ata8.00: status: { DRDY }
[138662.049018] ata8.00: failed command: READ FPDMA QUEUED
[138662.049884] ata8.00: cmd 60/58:08:00:1e:d1/00:00:4a:00:00/40 tag 1 ncq 45056 in
[138662.049885]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138662.051674] ata8.00: status: { DRDY }
[138662.052563] ata8.00: failed command: READ FPDMA QUEUED
[138662.053475] ata8.00: cmd 60/28:10:58:1e:d1/00:00:4a:00:00/40 tag 2 ncq 20480 in
[138662.053476]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138662.055352] ata8.00: status: { DRDY }
[138662.056266] ata8.00: failed command: READ FPDMA QUEUED
[138662.057230] ata8.00: cmd 60/80:18:00:1f:d1/00:00:4a:00:00/40 tag 3 ncq 65536 in
[138662.057231]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138662.059142] ata8.00: status: { DRDY }
[138662.060121] ata8: hard resetting link
[138662.061080] ata8: nv: skipping hardreset on occupied port
[138662.540147] ata8: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138663.010052] ata7: EH in SWNCQ mode,QC:qc_active 0x3 sactive 0x3
[138663.011086] ata7: SWNCQ:qc_active 0x1 defer_bits 0x2 last_issue_tag 0x0
[138663.011087]   dhfis 0x1 dmafis 0x0 sdbfis 0x0
[138663.013101] ata7: ATA_REG 0x40 ERR_REG 0x0
[138663.014125] ata7: tag : dhfis dmafis sdbfis sacitve
[138663.015123] ata7: tag 0x0: 1 0 0 1  
[138663.016133] ata7.00: exception Emask 0x0 SAct 0x3 SErr 0x0 action 0x6 frozen
[138663.017172] ata7.00: failed command: READ FPDMA QUEUED
[138663.018211] ata7.00: cmd 60/b0:00:0f:3b:31/00:00:13:00:00/40 tag 0 ncq 90112 in
[138663.018212]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138663.020278] ata7.00: status: { DRDY }
[138663.021335] ata7.00: failed command: READ FPDMA QUEUED
[138663.022419] ata7.00: cmd 60/80:08:3f:3c:31/00:00:13:00:00/40 tag 1 ncq 65536 in
[138663.022420]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138663.024615] ata7.00: status: { DRDY }
[138663.025708] ata7: hard resetting link
[138663.026778] ata7: nv: skipping hardreset on occupied port
[138663.500155] ata7: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138667.560111] ata8.00: qc timeout (cmd 0x27)
[138667.561238] ata8.00: failed to read native max address (err_mask=0x4)
[138667.562305] ata8.00: HPA support seems broken, skipping HPA handling
[138667.563363] ata8.00: revalidation failed (errno=-5)
[138667.564423] ata8: hard resetting link
[138667.565471] ata8: nv: skipping hardreset on occupied port
[138668.050152] ata8: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138668.130248] ata8.00: configured for UDMA/133
[138668.131306] ata8.00: device reported invalid CHS sector 0
[138668.132355] ata8.00: device reported invalid CHS sector 0
[138668.133383] ata8.00: device reported invalid CHS sector 0
[138668.134398] ata8.00: device reported invalid CHS sector 0
[138668.135405] ata8: EH complete
[138668.521316] ata7.00: qc timeout (cmd 0x27)
[138668.522349] ata7.00: failed to read native max address (err_mask=0x4)
[138668.523331] ata7.00: HPA support seems broken, skipping HPA handling
[138668.524321] ata7.00: revalidation failed (errno=-5)
[138668.525290] ata7: hard resetting link
[138668.526246] ata7: nv: skipping hardreset on occupied port
[138669.010141] ata7: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138669.090268] ata7.00: configured for UDMA/133
[138669.091215] ata7.00: device reported invalid CHS sector 0
[138669.092170] ata7.00: device reported invalid CHS sector 0
[138669.093112] ata7: EH complete
[138698.953877] ata8: EH in SWNCQ mode,QC:qc_active 0xF sactive 0xF
[138698.954847] ata8: SWNCQ:qc_active 0x1 defer_bits 0xE last_issue_tag 0x0
[138698.954848]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138698.956684] ata8: ATA_REG 0x40 ERR_REG 0x0
[138698.957598] ata8: tag : dhfis dmafis sdbfis sacitve
[138698.958511] ata8: tag 0x0: 0 0 0 1  
[138698.959418] ata8.00: exception Emask 0x0 SAct 0xf SErr 0x0 action 0x6 frozen
[138698.960342] ata8.00: failed command: READ FPDMA QUEUED
[138698.961264] ata8.00: cmd 60/80:00:00:1f:d1/00:00:4a:00:00/40 tag 0 ncq 65536 in
[138698.961265]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138698.963150] ata8.00: status: { DRDY }
[138698.964102] ata8.00: failed command: READ FPDMA QUEUED
[138698.965069] ata8.00: cmd 60/28:08:58:1e:d1/00:00:4a:00:00/40 tag 1 ncq 20480 in
[138698.965070]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138698.966959] ata8.00: status: { DRDY }
[138698.967892] ata8.00: failed command: READ FPDMA QUEUED
[138698.968822] ata8.00: cmd 60/58:10:00:1e:d1/00:00:4a:00:00/40 tag 2 ncq 45056 in
[138698.968823]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138698.970715] ata8.00: status: { DRDY }
[138698.971662] ata8.00: failed command: READ FPDMA QUEUED
[138698.972592] ata8.00: cmd 60/80:18:80:1d:d1/00:00:4a:00:00/40 tag 3 ncq 65536 in
[138698.972593]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138698.974509] ata8.00: status: { DRDY }
[138698.975471] ata8: hard resetting link
[138698.976405] ata8: nv: skipping hardreset on occupied port
[138699.460127] ata8: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138699.524011] ata8.00: configured for UDMA/133
[138699.524956] ata8.00: device reported invalid CHS sector 0
[138699.525898] ata8.00: device reported invalid CHS sector 0
[138699.526826] ata8.00: device reported invalid CHS sector 0
[138699.527734] ata8.00: device reported invalid CHS sector 0
[138699.528633] ata8: EH complete
[138699.980136] ata7: EH in SWNCQ mode,QC:qc_active 0x3 sactive 0x3
[138699.981106] ata7: SWNCQ:qc_active 0x1 defer_bits 0x2 last_issue_tag 0x0
[138699.981107]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138699.983001] ata7: ATA_REG 0x40 ERR_REG 0x0
[138699.983950] ata7: tag : dhfis dmafis sdbfis sacitve
[138699.984890] ata7: tag 0x0: 0 0 0 1  
[138699.985835] ata7.00: exception Emask 0x0 SAct 0x3 SErr 0x0 action 0x6 frozen
[138699.986787] ata7.00: failed command: READ FPDMA QUEUED
[138699.987735] ata7.00: cmd 60/80:00:3f:3c:31/00:00:13:00:00/40 tag 0 ncq 65536 in
[138699.987736]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138699.989667] ata7.00: status: { DRDY }
[138699.990625] ata7.00: failed command: READ FPDMA QUEUED
[138699.991601] ata7.00: cmd 60/b0:08:0f:3b:31/00:00:13:00:00/40 tag 1 ncq 90112 in
[138699.991602]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138699.993602] ata7.00: status: { DRDY }
[138699.994595] ata7: hard resetting link
[138699.995569] ata7: nv: skipping hardreset on occupied port
[138705.520105] ata7: link is slow to respond, please be patient (ready=0)
[138710.021352] ata7: SRST failed (errno=-16)
[138710.022395] ata7: hard resetting link
[138710.023375] ata7: nv: skipping hardreset on occupied port
[138710.510130] ata7: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138710.570285] ata7.00: configured for UDMA/133
[138710.571279] ata7.00: device reported invalid CHS sector 0
[138710.572337] ata7.00: device reported invalid CHS sector 0
[138710.573367] ata7: EH complete
[138729.950136] ata8: EH in SWNCQ mode,QC:qc_active 0xF sactive 0xF
[138729.951134] ata8: SWNCQ:qc_active 0x1 defer_bits 0xE last_issue_tag 0x0
[138729.951135]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138729.953034] ata8: ATA_REG 0x40 ERR_REG 0x0
[138729.953974] ata8: tag : dhfis dmafis sdbfis sacitve
[138729.954923] ata8: tag 0x0: 0 0 0 1  
[138729.955847] ata8.00: exception Emask 0x0 SAct 0xf SErr 0x0 action 0x6 frozen
[138729.956790] ata8.00: failed command: READ FPDMA QUEUED
[138729.957720] ata8.00: cmd 60/80:00:80:1d:d1/00:00:4a:00:00/40 tag 0 ncq 65536 in
[138729.957721]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138729.959606] ata8.00: status: { DRDY }
[138729.960540] ata8.00: failed command: READ FPDMA QUEUED
[138729.961468] ata8.00: cmd 60/58:08:00:1e:d1/00:00:4a:00:00/40 tag 1 ncq 45056 in
[138729.961469]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138729.963367] ata8.00: status: { DRDY }
[138729.964321] ata8.00: failed command: READ FPDMA QUEUED
[138729.965275] ata8.00: cmd 60/28:10:58:1e:d1/00:00:4a:00:00/40 tag 2 ncq 20480 in
[138729.965276]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138729.967215] ata8.00: status: { DRDY }
[138729.968216] ata8.00: failed command: READ FPDMA QUEUED
[138729.969231] ata8.00: cmd 60/80:18:00:1f:d1/00:00:4a:00:00/40 tag 3 ncq 65536 in
[138729.969232]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138729.971288] ata8.00: status: { DRDY }
[138729.972310] ata8: hard resetting link
[138729.973315] ata8: nv: skipping hardreset on occupied port
[138730.460146] ata8: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138730.520269] ata8.00: configured for UDMA/133
[138730.521301] ata8.00: device reported invalid CHS sector 0
[138730.522322] ata8.00: device reported invalid CHS sector 0
[138730.523323] ata8.00: device reported invalid CHS sector 0
[138730.524310] ata8.00: device reported invalid CHS sector 0
[138730.525287] ata8: EH complete
[138740.982624] ata7: EH in SWNCQ mode,QC:qc_active 0x3 sactive 0x3
[138740.983631] ata7: SWNCQ:qc_active 0x1 defer_bits 0x2 last_issue_tag 0x0
[138740.983632]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138740.985533] ata7: ATA_REG 0x40 ERR_REG 0x0
[138740.986463] ata7: tag : dhfis dmafis sdbfis sacitve
[138740.987402] ata7: tag 0x0: 0 0 0 1  
[138740.988339] ata7.00: exception Emask 0x0 SAct 0x3 SErr 0x0 action 0x6 frozen
[138740.989293] ata7.00: failed command: READ FPDMA QUEUED
[138740.990248] ata7.00: cmd 60/b0:00:0f:3b:31/00:00:13:00:00/40 tag 0 ncq 90112 in
[138740.990249]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138740.992194] ata7.00: status: { DRDY }
[138740.993181] ata7.00: failed command: READ FPDMA QUEUED
[138740.994143] ata7.00: cmd 60/80:08:3f:3c:31/00:00:13:00:00/40 tag 1 ncq 65536 in
[138740.994144]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138740.996115] ata7.00: status: { DRDY }
[138740.997110] ata7: hard resetting link
[138740.998090] ata7: nv: skipping hardreset on occupied port
[138741.480070] ata7: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138741.540182] ata7.00: configured for UDMA/133
[138741.541207] ata7.00: device reported invalid CHS sector 0
[138741.542239] ata7.00: device reported invalid CHS sector 0
[138741.543251] ata7: EH complete
[138760.950132] ata8: EH in SWNCQ mode,QC:qc_active 0xF sactive 0xF
[138760.951150] ata8: SWNCQ:qc_active 0x1 defer_bits 0xE last_issue_tag 0x0
[138760.951151]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138760.953083] ata8: ATA_REG 0x40 ERR_REG 0x0
[138760.954041] ata8: tag : dhfis dmafis sdbfis sacitve
[138760.955016] ata8: tag 0x0: 0 0 0 1  
[138760.955961] ata8.00: NCQ disabled due to excessive errors
[138760.956905] ata8.00: exception Emask 0x0 SAct 0xf SErr 0x0 action 0x6 frozen
[138760.957850] ata8.00: failed command: READ FPDMA QUEUED
[138760.958802] ata8.00: cmd 60/80:00:00:1f:d1/00:00:4a:00:00/40 tag 0 ncq 65536 in
[138760.958803]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138760.960710] ata8.00: status: { DRDY }
[138760.961644] ata8.00: failed command: READ FPDMA QUEUED
[138760.962589] ata8.00: cmd 60/28:08:58:1e:d1/00:00:4a:00:00/40 tag 1 ncq 20480 in
[138760.962590]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138760.964509] ata8.00: status: { DRDY }
[138760.965455] ata8.00: failed command: READ FPDMA QUEUED
[138760.966406] ata8.00: cmd 60/58:10:00:1e:d1/00:00:4a:00:00/40 tag 2 ncq 45056 in
[138760.966407]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138760.968336] ata8.00: status: { DRDY }
[138760.969290] ata8.00: failed command: READ FPDMA QUEUED
[138760.970252] ata8.00: cmd 60/80:18:80:1d:d1/00:00:4a:00:00/40 tag 3 ncq 65536 in
[138760.970253]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138760.972253] ata8.00: status: { DRDY }
[138760.973287] ata8: hard resetting link
[138760.974315] ata8: nv: skipping hardreset on occupied port
[138761.460072] ata8: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138761.520212] ata8.00: configured for UDMA/133
[138761.521230] ata8.00: device reported invalid CHS sector 0
[138761.522248] ata8.00: device reported invalid CHS sector 0
[138761.523255] ata8.00: device reported invalid CHS sector 0
[138761.524261] ata8.00: device reported invalid CHS sector 0
[138761.525257] ata8: EH complete
[138771.980130] ata7: EH in SWNCQ mode,QC:qc_active 0x3 sactive 0x3
[138771.981140] ata7: SWNCQ:qc_active 0x1 defer_bits 0x2 last_issue_tag 0x0
[138771.981141]   dhfis 0x0 dmafis 0x0 sdbfis 0x0
[138771.983081] ata7: ATA_REG 0x40 ERR_REG 0x0
[138771.984048] ata7: tag : dhfis dmafis sdbfis sacitve
[138771.984986] ata7: tag 0x0: 0 0 0 1  
[138771.985919] ata7.00: NCQ disabled due to excessive errors
[138771.986849] ata7.00: exception Emask 0x0 SAct 0x3 SErr 0x0 action 0x6 frozen
[138771.987800] ata7.00: failed command: READ FPDMA QUEUED
[138771.988749] ata7.00: cmd 60/80:00:3f:3c:31/00:00:13:00:00/40 tag 0 ncq 65536 in
[138771.988750]          res 40/00:ff:00:00:00/00:00:00:00:00/40 Emask 0x4 (timeout)
[138771.990675] ata7.00: status: { DRDY }
[138771.991651] ata7.00: failed command: READ FPDMA QUEUED
[138771.992647] ata7.00: cmd 60/b0:08:0f:3b:31/00:00:13:00:00/40 tag 1 ncq 90112 in
[138771.992648]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[138771.994622] ata7.00: status: { DRDY }
[138771.995607] ata7: hard resetting link
[138771.996572] ata7: nv: skipping hardreset on occupied port
[138772.480154] ata7: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[138772.540270] ata7.00: configured for UDMA/133
[138772.541275] ata7.00: device reported invalid CHS sector 0
[138772.542273] ata7.00: device reported invalid CHS sector 0
[138772.543249] ata7: EH complete