BE2015-Tape goes offline

Hi all,

I got a fresh BE2015 installation on a W2012R2.The first backupjob to a LTO6 drive finished correctly.

Since then, every further job is being canceled because of a storage or mount  error. The Tape goes offline.

This happens even with new tapes.

There is a hint to look in support DB V-275-1137, but I can´t find anything helpful. --> "no results found"

Any help is welcome.....