Before your reply I also tried to recompile and reinstall hplip manually by following: http://hplipopensource.com/hplip-web/install/manual/distros/debian.html. No good, same issue.

Did what you've requested:

$ sudo ps -ef | grep systray
[sudo] password for joao: 
joao      2050  1956  0 11:23 ?        00:00:00 /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4/panel/plugins/libsystray.so 6 20971559 systray Notification Area Area where notification icons appear 
joao      2380     1  0 11:25 ?        00:00:01 python /usr/bin/hp-systray --force-startup
joao      2381  2380  0 11:25 ?        00:00:00 python /usr/bin/hp-systray --force-startup
joao      2382  2381  0 11:25 ?        00:00:00 python /usr/bin/hp-systray --force-startup
joao      2673  2402  0 11:31 pts/0    00:00:00 grep systray

$ sudo kill 2380 2381 2382

$ sudo ps -ef | grep systray
joao      2050  1956  0 11:23 ?        00:00:00 /usr/lib/xfce4/panel/wrapper /usr/lib/xfce4/panel/plugins/libsystray.so 6 20971559 systray Notification Area Area where notification icons appear 
joao      2792  2402  0 11:32 pts/0    00:00:00 grep systray

Launch Xsane. Tried to scan from Flatbed. No problem. And then using ADF. It loads first adf sheet and then aborts with connection error, Xsane halts.

Edited bb_ledm.c, common.h and hpmudi.h, make and sudo make install as requested. Run hp-scan --adf, same issue.

/var/log/messages:

Oct  4 11:58:55 eeeboxpc python: hp-scan[10898]: warning: No destinations specified. Adding 'file' destination by default.
Oct  4 11:58:59 eeeboxpc python: hp-scan[10898]: warning: File destination enabled with no output file specified.
Oct  4 11:58:59 eeeboxpc python: hp-scan[10898]: warning: Defaulting to '/home/joao/hpscan001.pdf'.

/var/log/syslog:

Oct  4 11:58:55 eeeboxpc python: hp-scan[10898]: warning: No destinations specified. Adding 'file' destination by default.
Oct  4 11:58:58 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:58:59 eeeboxpc python: hp-scan[10898]: warning: File destination enabled with no output file specified.
Oct  4 11:58:59 eeeboxpc python: hp-scan[10898]: warning: Defaulting to '/home/joao/hpscan001.pdf'.
Oct  4 11:58:59 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:00 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:01 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:02 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:03 eeeboxpc python: scan/sane/http.c 344: invalid http_status=404
Oct  4 11:59:03 eeeboxpc python: scan/sane/http.c 348: dumping len=259
Oct  4 11:59:04 eeeboxpc python: io/hpmud/jd.c 624: timeout read_channel sec=1 hp:/net/Officejet_7500_E910?zc=HPA9B9D1
Oct  4 11:59:04 eeeboxpc python: scan/sane/http.c 188: timeout reading data sec_timeout=1
Oct  4 11:59:05 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:06 eeeboxpc python: hp-scan[10898]: error: SANE: Error during device I/O (code=9)
Oct  4 11:59:06 eeeboxpc python: io/hpmud/hpmud.c 214: io/hpmud/hpmud.c: service_to_channel(): sn=HP-LEDM-SCAN
Oct  4 11:59:07 eeeboxpc python: io/hpmud/hpmud.c 341: device_cleanup: device uri=hp:/net/Officejet_7500_E910?zc=HPA9B9D1
Oct  4 11:59:07 eeeboxpc python: io/hpmud/hpmud.c 353: device_cleanup: close device dd=1...
Oct  4 11:59:07 eeeboxpc python: io/hpmud/hpmud.c 355: device_cleanup: done closing device dd=1