I’ve seen issues like this before related to quick actions based on objects to which the current user does not have permission. This was due to a flaw in Salesforce’s Chatter Visualforce component - it didn’t check user permissions before attempting to pre-render some of the quick action forms, resulting in unexpected output from a REST service and causing a Javascript error which made the Chatter component non-functional.
I’ll update this post with additional information soon, but wanted to give you a head start on solving this.
