Its the condition.
(Owner of (Triggering unit)) == (Owner of (Leaving Unit))
There is neither a triggering unit nor a leaving unit.
The event (a player leaves the game) does not involve any units at all.
"Pick all units owned by triggering player and do remove picked unit"
is what you need.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.