The game got into a weird state where I could not summon anything anymore.
No Puppets, Air Elementals, Skeletons, Specters ... nothing.
I could cast other spells with the same mages/archmages/vampires, but just not summons.
To try to fix it, I deleted all my summoned creatures. I still could not do any summons. For purposes of testing the bug, we saved this game, and my opponent deleted his hordes of skeletons, and voila! I could summon again.
But, wait, while I was stuck and unable to summon, even before & after I deleted all of my summons, my opponent could still summon skeletons.
Saved game attached. Version 0.40, patch "c".
(Now that this game is finished, I'll upgrade, but I still report the bug cause I haven't seen anything like this reported as fixed.)