Partitioned library

We did not have problem with this untill we use the BE 2010 R3, this came in light since we have upgrade our BE ro BE 15.

Also in last 4 year or so I alyways put the media manually in correct media set (after erasing the tpaes) and we had never any problem.

I am not trying to argue, and I do not have an obvious explanation without more details, but I don't think I agree with that perspective. Maybe there is some subtle detail that I am missing here.  The most obvious answer would be if you had (for example) a 3 week overwrite protection time, you manually put the tapes into the media set, they then became overwrite protected for 3 weeks, then on week number 4, BE tried to use the tape as it was now overwritable.  I don't see that being likely in a 2 slot partition, but it certainly could happen in a larger partion/library with enough slots and spare media available.

A lot of things DID change between BE 2010 and BE 15, but tape usage and tape media sets protection rules wasn't one of the obvious ones.

Regardless, I think you are on the right track now and let's see what happens at the end of the month.