I've seen something similar to this before, I don't remember what it was though, I'll poke around a bit.
It seems that is only happening to you, everything is fine here.
Ah, I seem to have found it. If what I'm reading is correct someone sent you a PM and then revoked it from tracking.
It had happened a lot of time on UB. Perhaps what Paul said is the issue most probably.
It is more then likely a deleted user/staff member and their account was deleted revoking them from PM'ing you. it happens now and then.
We don't delete users here, only ban them
