Special:GlobalRenameRequest helpfully "suggests" a new username, which is the user's current username followed by three random characters:
do { $rand = $this->getUser()->getName() . rand( 123, 999 ); } while ( !GlobalRenameRequest::isNameAvailable( $rand )->isOK() );
If the user's current username is constructed such that none of those usernames can possibly pass isNameAvailable, this will loop forever. Presumably, this consumes 100% of a CPU core until the request times out.
I considered registering all of User:ST47123 .. User:ST47999, but I'm lazy and there is an easier way:
The new username must pass all of the tests in UserNameUtils.php, including that the new username must not be longer than MaxNameChars (which is 85 on WMF). If the current username is at least 83 characters long, this will loop forever. With sufficient threads, this could very easily DoS the site.
I would suggest removing this "suggested username" feature from GlobalRenameRequest entirely, as adding three digits to the end of the username isn't a very good suggestion anyway.
If a traceback would be helpful, you can find one here: [fc0657b2-77c2-41eb-9996-ce38f062394a] 2020-08-20 02:27:10: Fatal exception of type "WMFTimeoutException"