Two thoughts:
First the live server is dedicated to Live chess so the answer you got is to simply head back to the main part of the website. If you have done that and are still getting the message, then maybe you just need to clear your browser cache. This site is a bit of a memory hog and if the cache isn't regularly cleared, then strange things can happen. Good luck!
When I try to change my username I get this message:
Please wait until after your live chess session to change your username.'
I don't believe I've changed my username before, so I don't think it's that.
Any ideas?