sorry for the delay in getting back to you (local time here 8am).
Do you still have the issue? It's odd because that option only checks the number of super users and doesn't allow creation of new ones...
There are some mod_security rules that causes conflict changing configuration because info is storedin json format, but you only get a temporary 403 error message.
If you still have the issue please send me administrative credentials to access to the site's backend and also for cpanel (or ftp access). Use the email from which you receive forum notifications.