What causes my error "Illegal choice in Sorted by element"?
I was able to figure out what view was being used on group pages (The Commons Browsing Widget) and see that there were three sorting criteria. One of them is the Radioactivity module. 

I checked the general status report (admin/reports/status) to see that there was an issue with RadioActivity module. 

I removed this sorting feature for now, but it should work now that I cleared up the issues with the module. I guess no one could have known that this was the cause. I'm keeping this up in case anyone else finds it helpful.