Thought I'd finally got a work round for this last week, as I managed to achieve many succesful scheduled backups one after the other.
I utilise the bemcmd -o503 & -o502 commands from the BEX program folder - this is done via 2 batch files set up in Task Manager. The only way to get continued backups without this error was to stop/start the BEX services in between scheduled jobs.
This has now stopped working, & I'm well & truely at a frustrating halt with this now.
Tracer fully passes tests, as does HP Tape Tools - so it's not a hardware issue.
I've tried removing the device & letting it redetect, as seemed to fix it for other users, but not for me.
I have the following updates installed in my 12.5 installation:
SP2, 322898, 324947, 317436, 325647, 326004, 319242, 324012 & 323894.
With the time & effort spent on this product so far, I'm seriously thinking of ditching it for something else!!