Can not create or edit Managers, Administrators, Super Administrators using Admin Tools (403 error thrown)

Admin Tools Professional contains a feature called Web Application Firewall, or WAF for short. One of the security features applied by WAF is to block editing or adding Managers, Administrators and Super Administrators.

Please go to Components, Admin Tools and click on the Web Application Firewall button. In the new page, click on WAF Configuration. Find the Disable editing backend users' properties option and set it to No. Now you can create and edit all users on your site. For security reasons, remember to re-enable this option after you're done adding/editing users!