Looking in Firebug pulled an error that Enumerable does not exist, leftandmain.js. I found this thread:
So it only happens on Live sites without isDev on, that's exactly what happened to me. Cheers!
Noticed I can't change the group a user belongs to on a live site. If I click the tree dropdown the whole Member popup content just scrolls back to the top. 3 tabs are visible but the body of the popup doesn't split between the tabs, it's just one long scroll. Cliking tabs scrolls up and down. No errors appear.
Didn't customize member at all but do have FacebookConnect and MemberProfiles modules setup.