In older Versions it was allowed to have a Username with special characters. f.ex. MP:George
After updating to beta1 version i wanted to change some settings in the userprofile but this does not save since the username is checked again.
Solution:
when editing my existing userprofile there is no need to check the username before saving since i can“t change it anyway
In admin_function.php around line 500 wrap if(!validate_username).. with if like the following:
if($update==false){
if ( !validate_username( $user->user_login ) )
$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is invalid. Please enter a valid username.' ));
}