HP DP Basic Express - server hangs

[SCHK]

Hi, we have the above s/w, version 5.0.00.59287 running on Win 2K8 R2 SP1 (All patched up).
The Server is HP, the internal tape station (LTO 4) and the tapes are all HP-ware.

Doing the daily single tape backups w/validation works like a charm.

The weekly backups use 2 tapes, backup to tape 1 and 2 is OK, but when validating the backup, the server suddenly hangs completely, not responding to anything but a power cycle.
I will be grateful for any kind of help or hints.