I try to delete a user and get this message "This user can't be removed cause he needs to be in at least one team."

This message is displayed under circumstances if you are a team admin and try to remove a user from your team. 

SIGNL4 currently does not support team-less users. So, when you try to remove a user from your team and this user is not a member of another team, this error message is displayed.

Solution:

As admin rights of a team admin are restricted, your global admin needs to remove the user from your team, either by deleting it entirely or by assigning the user to a different team first, followed by the removal from your team.