Got it again. I have no idea what triggered it. Here are extracts from log files:

messages (135Mb):

Jul 21 18:31:00 localhost gconfd (foxy-4016): Resolved address "xml:readonly:/etc/gconf/gconf.xml.mandatory" to a read-only configuration source at position 0
Jul 21 18:31:00 localhost gconfd (foxy-4016): Resolved address "xml:readwrite:/home/alex/.gconf" to a writable configuration source at position 1
Jul 21 18:31:00 localhost gconfd (foxy-4016): Resolved address "xml:readonly:/etc/gconf/gconf.xml.defaults" to a read-only configuration source at position 2
Jul 21 18:31:00 localhost gconfd (foxy-4016): Resolved address "xml:readonly:/var/lib/gconf/debian.defaults" to a read-only configuration source at position 3
Jul 21 18:31:00 localhost gconfd (foxy-4016): Resolved address "xml:readonly:/var/lib/gconf/defaults" to a read-only configuration source at position 4
Jul 21 18:53:03 localhost -- MARK --
Jul 21 19:13:03 localhost -- MARK --
Jul 21 19:33:04 localhost -- MARK --
Jul 21 19:53:04 localhost -- MARK --
Jul 21 19:53:05 localhost kernel: ge Reference Count (7FB1) in object e11bdc2c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc54 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc7c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdca4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdd44 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object eccceefc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd8e4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object ecccef4c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd9ac [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd6b4 [20060707]

kernel.log.0 (135Mb):

Jul 21 16:56:38 localhost kernel: [ 3946.023000] end_request: I/O error, dev sr0, sector 0
Jul 21 19:53:05 localhost kernel: ge Reference Count (7FB1) in object e11bdc2c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc54 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc7c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdca4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdd44 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object eccceefc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd8e4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object ecccef4c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd9ac [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd6b4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FAF) in object e11bd8bc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB4) in object e11bd95c [20060707]

syslog.0 (135M)
Jul 21 18:57:20 localhost smbd[11808]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 18:57:20 localhost smbd[11811]: [2007/07/21 18:57:20, 0] smbd/map_username.c:map_username(107) 
Jul 21 18:57:20 localhost smbd[11811]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 18:57:21 localhost smbd[11815]: [2007/07/21 18:57:21, 0] smbd/map_username.c:map_username(107) 
Jul 21 18:57:21 localhost smbd[11815]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 18:57:24 localhost smbd[11831]: [2007/07/21 18:57:24, 0] smbd/map_username.c:map_username(107) 
Jul 21 18:57:24 localhost smbd[11831]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:01:42 localhost modprobe: WARNING: Not loading blacklisted module ipv6 
Jul 21 19:06:13 localhost nmbd[12477]: [2007/07/21 19:06:13, 0] nmbd/nmbd_browsesync.c:find_domain_master_name_query_fail(351) 
Jul 21 19:06:13 localhost nmbd[12477]:   find_domain_master_name_query_fail: 
Jul 21 19:06:13 localhost nmbd[12477]:   Unable to find the Domain Master Browser name WORKGROUP<1b> for the workgroup WORKGROUP. 
Jul 21 19:06:13 localhost nmbd[12477]:   Unable to sync browse lists in this workgroup. 
Jul 21 19:12:35 localhost smbd[13765]: [2007/07/21 19:12:35, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:12:35 localhost smbd[13765]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:12:35 localhost smbd[13768]: [2007/07/21 19:12:35, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:12:35 localhost smbd[13768]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:12:36 localhost smbd[13772]: [2007/07/21 19:12:36, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:12:36 localhost smbd[13772]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:12:36 localhost smbd[13781]: [2007/07/21 19:12:36, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:12:36 localhost smbd[13781]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:17:01 localhost /USR/SBIN/CRON[14325]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jul 21 19:21:20 localhost nmbd[12477]: [2007/07/21 19:21:20, 0] nmbd/nmbd_browsesync.c:find_domain_master_name_query_fail(351) 
Jul 21 19:21:20 localhost nmbd[12477]:   find_domain_master_name_query_fail: 
Jul 21 19:21:20 localhost nmbd[12477]:   Unable to find the Domain Master Browser name WORKGROUP<1b> for the workgroup WORKGROUP. 
Jul 21 19:21:20 localhost nmbd[12477]:   Unable to sync browse lists in this workgroup. 
Jul 21 19:27:51 localhost smbd[15669]: [2007/07/21 19:27:51, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:27:51 localhost smbd[15669]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:27:51 localhost smbd[15672]: [2007/07/21 19:27:51, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:27:51 localhost smbd[15672]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:27:51 localhost smbd[15676]: [2007/07/21 19:27:51, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:27:51 localhost smbd[15676]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:27:52 localhost smbd[15685]: [2007/07/21 19:27:52, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:27:52 localhost smbd[15685]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:31:42 localhost modprobe: WARNING: Not loading blacklisted module ipv6 
Jul 21 19:36:28 localhost nmbd[12477]: [2007/07/21 19:36:28, 0] nmbd/nmbd_browsesync.c:find_domain_master_name_query_fail(351) 
Jul 21 19:36:28 localhost nmbd[12477]:   find_domain_master_name_query_fail: 
Jul 21 19:36:28 localhost nmbd[12477]:   Unable to find the Domain Master Browser name WORKGROUP<1b> for the workgroup WORKGROUP. 
Jul 21 19:36:28 localhost nmbd[12477]:   Unable to sync browse lists in this workgroup. 
Jul 21 19:43:06 localhost smbd[17648]: [2007/07/21 19:43:06, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:43:06 localhost smbd[17648]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:43:06 localhost smbd[17651]: [2007/07/21 19:43:06, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:43:06 localhost smbd[17651]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:43:06 localhost smbd[17655]: [2007/07/21 19:43:06, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:43:06 localhost smbd[17655]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:43:07 localhost smbd[17664]: [2007/07/21 19:43:07, 0] smbd/map_username.c:map_username(107) 
Jul 21 19:43:07 localhost smbd[17664]:   can't open username map /etc/samba/smbusers. Error No such file or directory 
Jul 21 19:51:28 localhost nmbd[12477]: [2007/07/21 19:51:28, 0] nmbd/nmbd_browsesync.c:find_domain_master_name_query_fail(351) 
Jul 21 19:51:28 localhost nmbd[12477]:   find_domain_master_name_query_fail: 
Jul 21 19:51:28 localhost nmbd[12477]:   Unable to find the Domain Master Browser name WORKGROUP<1b> for the workgroup WORKGROUP. 
Jul 21 19:51:28 localhost nmbd[12477]:   Unable to sync browse lists in this workgroup. 
Jul 21 19:53:05 localhost kernel: ge Reference Count (7FB1) in object e11bdc2c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc54 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdc7c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdca4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bdd44 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object eccceefc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd8e4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object ecccef4c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd9ac [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB1) in object e11bd6b4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FAF) in object e11bd8bc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB4) in object e11bd95c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bdc2c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bdc54 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bdc7c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bdca4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bdd44 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object eccceefc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bd8e4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object ecccef4c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bd9ac [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB2) in object e11bd6b4 [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB0) in object e11bd8bc [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB5) in object e11bd95c [20060707]
Jul 21 19:53:05 localhost kernel: [14534.715000] ACPI Warning (utdelete-0397): Large Reference Count (7FB3) in object e11bdc2c [20060707]