OK, found the culprit: it's the changed logic LL implemented last week in an attempt to fix their capability-based offline IMs retrieval code, and that I backported with too much enthusiasm/hope.
Note that this does NOT apply to offline IMs gifts (thus why I could not reproduce it with group IMs), but to objects given by scripted objects.
This will be reverted in next release.
If you compile the viewer yourself, you may revert (with 'patch -p1 -s -R <path-to-patch/bogus_im_patch.txt' from inside the linden/ directory of the sources) this patch: