Can't add new users

I’m in 108.6 (hass.io on intel nuc) and always just had myself as a user (+some system generated ones like hass.io, homeassistant cast, homeassistant cloud and legacy API).

Now I want to add another user with the objective to create their own lovelace interface.

However when I want to create a user (via configuration>users) I get “unknown error” , nothing else to give me any pointers.

Any ideas on how to troubleshoot this or what the reason is?

Is your existing user an administrator?

Yes it is!

Solved: