The hospital has some dental X-Ray machine by Villa Sistemi Medicali. A typical UID from it:
1.2.840.14-2PS-9091.1002.201409240811160965.1506952
(14-2PS-9091 is patient ID. Probably the manufacturer so far didn't encounter customers who need some additional coding in this value.)
The resulting UID is surely invalid, however PacsOne 6.4.1 receives those images from network instead of rejecting. Now local admins are unable to find them as the web interface disallows searching for invalid UIDs.
In that installation I simply commented out corresponding lines. But, if such a restriction is beneficial in the web interface, then it should be of the same kind as in PacsOne.exe. It's the inconsistent behavior that makes this a bug.