I regularly backup my Windows (7, 8, 8.1) machines to a Samba share. After an upgrade to 4.0.12 (from 3.6.x), the backups started failing. I downgraded the server and sure enough they started working again.
Through trial and error I found that setting 'server max protocol = SMB2_02' allows the backups to run against Samba 4. SMB2_10 or high causes a failure.
To reproduce, from a Windows box, run:
'wbadmin start backup -backupTarget:\\servername\share -allCritical'
With SMB2_10 or high, it will fail before reaching 1% after system image creation starts.
I have repeated this failure on 2x Debian Unstable servers. Client logs (on both sides) show no meaningful errors.
Just found this is a duplicate of https://bugzilla.samba.org/show_bug.cgi?id=10159
It may be nice to default the protocol to SMB2_02 in 4.x until this is resolved? I lost half a day trying to solve this, the root cause being an incompletely implemented protocol and not enough messaging on client nor server.
*** This bug has been marked as a duplicate of bug 10159 ***