500 error when trying to view individual results

Whenever a user on my site tries to view their own results they get a 500 error. This includes the admin account. It was working before, so I'm not sure what the deal is, as I haven't changed anything major.

These paths don't work for my administrator / any account:
http://site.com/quizfolder/?q=user/quiz/1/userresults
http://site.com/quizfolder/?q=node/1/myresults/1

This does show the results:
http://site.com/quizfolder/?q=node/1/results/1

But only the administrator can view results in this way.

Any ideas what's going on? Thanks!