Actually inventory and eject are the only option I can select, all the others are grey out.
But even if I try to eject a tape the monitor show the "...no idle device ..." messages.
This happens with any job runing against the drive (backups included).
As I said in the first post, backup to disk folder works fine.
After doing what you suggested, and reinstalling again the device drivers (tried with both veritas and tandberg) and rebooting the server, everything keeps the same.
I've got the same hardware configuration working in another box (same software as well).