What part of the user profile are you trying to update when it gives you the error. I made a minor change to my Music section and it passed through.
Signature but I tried Music now and it also broke forums (and doesn't update).
Also broke with Machina but Idk what he was trying to update.
It was Favourite Games, and I've just tested it out again by trying to change Favourite Games and Favourite Books at the same time.
I get the same problem I got originally - clicking 'Update Profile' doesn't seem to register, then when I try to browse the forums or go back to my profile I can't (I just get an endless loading animation on the tab and the page itself never updates). Clearing the site's cookies allows me to browse the forums and my profile again, though the changes made to the profile don't show up.
Could it be related to the sign-up issue that we're asking @jlauro about right now @TruckOSaurus ?