I get the age old message...
To allow everyone fair access to the limited cpu resources, each user may only be in one fight at a time.
If you clicked on fight before your last one finished, you will get this error. Remember, 'your last one' could have been a challenge page, so moving to another activity is NOT safe at this point.
This is open to abuse since it's so easy to create new user accounts. Creating multiple user accounts is illegal, but rather than make that process more onerous for legitimate new users (such as by email verification, which in any case is trivial to spoof), we've chosen as the most foolproof method to additionally limit fighting on an IP address basis for non-Supporters. This means that if you're behind a firewall that makes multiple computers appear to share the same IP, your fights will conflict with others from behind your firewall.
rather than click on Home, I clicked on the only other linked word: Supporters....Page Not Found.