This time I searched the archive before posting this issue.

Guess what happens when you try to access a labelled tape on an LDEV# >
255? Try:

SYSTEM ABORT 2559 FROM SUBSYSTEM 99
COMPATIBILITY MODE SUDDENDEATH 8606
SYSTEM HALT 7, $09FF

HP says:

* MPE/iX's labelled tape facility is STILL in compatibility mode, as of the
latest versions.
* HP has no plans to convert labelled tape handling to Native Mode.

Even more interesting is the fact that the customer has HP's Critical
System Support ($$$) and HP configured this system with DLT drives as LDEVs
1710, 1711, 1712. And wouldn't you know, I stepped on their land mine.

We can only work around this issue: reconfigure the drives to LDEV#'s <=
255; stop using labelled tapes. But right now the customer is in a Y2K
lockdown, so we're probably faced with the Henny Youngman school of problem
resolution (Doc, it hurts when I do this. Ok, don't do that!).

Cheers!

Scott