XF 1.1 Profiles throwing 500 error?

We've been running Xenforo for about a month now, smooth sailing for the most part.

Most recently however we noticed two user profiles (that we've been able to catch so far) throwing 500 errors for no apparent reason:


If I go into the CP and edit the profiles, it all shows and edits OK.

Other profiles, threads and pages are running fine. I've tried repairing the entire Xenforo DB and also rebuilding the user cache on the XF panel but nothing, still 500 on those two profiles.

What else could I check on or try to narrow down the issue?

Thank you.