"Sorry, but this username has already been taken."

I'm pretty desperate here, because this issue is causing my forum to fail.

Many of my members are having major problems with trying to register to my forum. http://everythingimvu.forumotion.com/ 


Whenever they try to sign up they get this message Sorry, but this username has already been taken.
So what i believe the problem might be, is that these usernames they're trying to sign up with, are taken on other forum sites. I don't understand why that would be interfering with my site, but i'd really appreciate it if someone would take the time to help me out.

No its not that they are taken on other forum sites each site can have the same name, like on my personal site, and the help site my names are the same.
now back to your issue it could be that it is thought of as a spam name and you may have the option for blocking the most commonly used spam names turned on.
ACP>General>Forum>security
Unauthorize subscriptions made with the most commonly used spam usernames. :No.
Save