This is not normal. It sounds like Admin Tools has some trouble writing the new Super Users list into the database. Please go to Components, Admin Tools, Web Application Firewall, Configure WAF and click on the Joomla! Feature Hardening Options tab. Find the "Monitor Super User accounts" option and set it to No. Click on Save & Close. Log out and log back in again (as any Super User).
Normally you should be able to re-enable this feature without having the other Super User blocked.
Alternatively, you can leave that setting off if you do not want Admin Tools to automatically block Super Users which were added outside the Joomla! administrator area.
Nicholas K. Dionysopoulos
Lead Developer and Director
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