Hi Henk!
This sounds like APAR IX84765:

IX84765 IF A VOLUME CONTAINS FILES WHICH COULD NOT BE RECLAIMED, ADSM SERVER

3.1.2.0 WILL DELETE THE VOLUME FROM THE STORAGEPOOL.

 If a volume contains files which could not be reclaimed, ADSM
 Server 3.1.2.0 will delete the volume from the storagepool.
 Messages from the actlog:
 ANR1175W Volume xxxx contains files which could not be reclaimed
 ANR1410W Access mode for volume xxxx now set to "unavailable".
 ANR1041I Space reclamation ended for volume xxxx.
 ANR1341I Scratch volume xxxx has been deleted from storage pool.

This should be fixed in level 3.1.2.20
Kindest regards,
Eric van Loon
KLM Royal Dutch Airlines