Whoops! An error occurred and we were unable to change your profile name. Are you sure you have entered the correct password? If the problem persists, please try again later.

This message came up to me when I entered a name that was already taken but I pressed change before it could say "This name is already taken". I'm not sure if it is a bug, but that could have been what happened to you.

Do you still have this issue? If not, how could you solve it?

Is this still an issue? Did you follow the username guidelines/did the page said the username was or wasn't available? We need more info for the bug to reproduce so any details that you remember will be helpful. 

The question is not only for the reporter, by the way, if any of the commenters reproduce it still please provide some more info.