admin "Access denied" when changing/adding users

[lucyhilltop]

What happened:

• Tried to change my user authorities, everything was fine
• Logged in from another computer to see, if the error the user reported was user-based => got an 'access denied' error
• Went back to my own computer => now also get the 'access denied' error when going through the exact same steps as before

Discovered I also cannot access /logs, /health and /metrics add-ons for admins. Creating vacations for other users and for myself works fine.

Logs were posted to vacation-tracker chat, as this is public github repository.