I have also tried verifying the steam cache. I will try disabling the AV now and see if that works.
May 9 2016
I don't have ZoneAlarm and saw that solution on a steam forum post.
I have this issue, and because I disabled steam in-game overlay I can't use the steam screenshot feature (which I do use a lot for other games). I have only disabled the in-game overlay due to it apparently causing crashes for some people.
Is there a built in screenshot feature at all?