Thanks [NAME] for your detailed answer. Meanwhile our problem has been solved. First thing I've found, was that the tape storage node (bagsv32) was not listed under StorageNodeProperties -> Configuration -> Advanced Devices -> Clone storage nodes. After adding it there it was possible again to use the tape devices. Second source of the trouble was a parallel installed networker client on the tape storage node (in addition with a different version). Since I have removed it, everything runs fine again. Thanks to all the supporters in this thread for helping me! Nico  
We have some trouble with a tape library managed by networker V19.4. and ealier versions. After clone to tape jobs to a tape pool are finished, networker will not mount tapes from another tape pool to the used drive. If you mount a tape from the desired pool manually, the waiting job will start. This happens with or without pool to target device selections. Can someone help? Nico
Networker not mounting tapes from different pools