New backup job - session setup failed: SUCCESS - 0

I have an existing backup between RN Ultra 2+ and a Win7/64 machine. It last worked yesterday

INCREMENTAL Backup started. Mon Nov  4 18:05:15 EST 2013

Job: 001
Protocol: cifs
Source: //xxx.xxx.xx.xx/d/Java/XXXXX
Destination: [work]//Java/XXXXX

Backup finished. Mon Nov  4 18:05:15 EST 2013

I set up a new backup to same machine and tried to test the connection, but it gave error connecting. I saved the backup job and ran it and it logs this

INCREMENTAL Backup started. Tue Nov  5 12:50:06 EST 2013

Job: 002
Protocol: cifs-ab
Source: //xxx.xxx.xx.xx/d/XXXXX
Destination: [work]//XXXXX

session setup failed: SUCCESS - 0
Files copied: 0

Backup finished. Tue Nov  5 12:50:06 EST 2013

I've not added/installed anything since yesterday. No updates. If I edit the backup job that worked yesterday and test the connection it also fails...

I monitored the connection with Wireshark and there's an SMB negotiate protocol request/response pair, which appears to return STATUS_SUCCESS
Can't find anything in any logs on the NAS that shows anything useful
Any ideas?