First of all thank you for your help and for patience reading my poor English. I followed your suggestion and performed another parity check: success, no error. During the previous parity check a got a lot of corrections on parity disk, but no errors in error column at all.
This morning I downloaded the syslog, and found out some strange things. It seems that I have some problems reading the usb flash stick some lines like:
Sep 25 09:53:13 Unraid101 kernel: read_file: error 2 opening /boot/config/super.dat
Sep 25 09:53:13 Unraid101 kernel: md: could not read superblock from /boot/config/super.dat
Sep 25 09:53:13 Unraid101 kernel: md: initializing superblock
and
Sep 25 09:53:23 Unraid101 emhttp: Start OK!
Sep 25 09:53:24 Unraid101 emhttp: unclean shutdown detected
Sep 25 09:53:24 Unraid101 kernel: mdcmd (46): check CORRECT
Sep 25 09:53:24 Unraid101 kernel: md: recovery thread woken up ...
Sep 25 09:53:24 Unraid101 kernel: md: recovery thread checking parity...
Sep 25 09:53:25 Unraid101 kernel: md: using 1536k window, over a total of 976762552 blocks.
Sep 25 09:53:25 Unraid101 kernel: md: correcting parity, sector=21048
Sep 25 09:53:25 Unraid101 kernel: md: correcting parity, sector=21056
Sep 25 09:53:25 Unraid101 kernel: md: correcting parity, sector=21064
Sep 25 09:53:25 Unraid101 kernel: md: correcting parity, sector=21072
anyway I attach the complete syslog, if you have some spare time, you could suggest me what to check, to avoid future problems.
Thank you
syslog-2015-09-25.zip