Hi,
I had a similar experience , do not know why but these tape drive are not compatible with DP 8.13 ,
In the end I did the following , install the MA server with DP7 and fixed or on servers that are MA .
Now, the problem occurs in all sessions ? Or only some servers ?
You can isolate that customers and test if backups of those clients can run for another MA .
My client had no contract so you could not open a case , if you have contract open one, I guesse there are bug with such situations
Regards