Not sure how to/if I'm supposed to, add this to Uber Crash Reports so I'll just put in here. http://crashes.uberent.com/report/index/bp-1f835ad6-ee57-43f5-a40f-146c02150821
I like how it doesn't say which effect it couldn't find. Anyway, try verifying the integrity of the game cache in Steam. Instructions can be found here: https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335
I believe that error message is actually reporting the effect that's missing; that effect is named "". No idea what would cause something to try to use an unnamed effect. Have you tried disabling PAStats? It shouldn't be the cause, but it's good to remove all variables.