Hi Kyle,
Yep that's what I first thought but nothing has changed in the Joomla config for the user groups and the only thing that's different is the new CB redirect plugin. So I disabled it and still the same results!
It's beginning to drive me insane as I've also just installed 3 older backups of the same site on a testing server (J 1.7.4 and CB 1.7.1) and they are working fine. So it appears to be something now to do with J 2.5.3 or the new plugin (even though it was disabled?)...
I have double-checked the permissions and the published Profile menu item and they are identical as far as I can see to the testing server versions. I need to try and get this working on 2.5.3 so I guess I'll keep trying..