Henrik -

I don't have a direct answer for you; but my suspicion is that drive
microcode is involved, perhaps in conjunction with cartridge vs. drive
characteristics.  The IBM readme document S1003097 for LTO4 shows a
very large number of problems that have had to be corrected,
suggestive of more, ongoing.  The stuck tape situation may suggest a
problem managing the mechanics and/or timing of drive operations.
Given the mechanical malfunction which results, I'd have your CE
attend to the next event, to examine the conditions logged in the
drive's memory, to try to uncover the cause.  The inability of an OS
command to effect tape dismount indicates a serious problem.  If the
CE can't be gotten in time during the next event, see if you can have
the operator get at the drive to record any error codes on its panel.
Make sure the CE pores over the sense bytes you're getting from the
drive.

   Richard Sims