If the stacking Mageslayer debuff on a target interrupts a spellcast the spell still goes in the turn queue but since it was still interrupted it never resolves meaning you have to load the game or hard quit. To see if the game would resolve this I left this combat encounter running for 2 hours and nothing happened, and I was able to recreate it again today against the same encounter. 

you should repost this in either the main bug report forum or the turn-based feedback forum, since i don't think this forum is actively monitored unless there is an actual patch on the beta branch being evaluated.

Noted, thanks.