Fromt he maintenance prompt, enter "dbms check full" and let it complete, it probably won't fix your problem but it can't hurt, then I would try stopping the vmail talks and restarts them...
From the serial port (or ssh terminal) enter:
iPVM_Stop
then when the command prompt returns enter
iPVM_Start
then when the command prompt returns, try the backup again... I have a system on MCD 4.2 that does something like this all the time, and although the log messages are slightly different, the problems we see and yours all seem to be related to free space on /vmail/ partition (yours in c/voxdrv/, mine problem is in /vmail/temp) and there isn't actually a problem, just something stuck in the iPVM tasks.
No guarantee it will work, but it is the only thing I can think of.
On another note, when did you last get a good backup? if it is fairly recent and the above stuff doesn't work I would recommend defaulting (or better yet a manual reinstall of software using the FixPartition flag), relicense and then restoring the database