Hi Klas,
thanks very much for the hint! - Very helpful, and it brought the solution.-
Just noticed that I had to set the User activation field to an empty string to activate the user. If it is not active it contains a kind of verification key. - In addition to that I do hide the radio buttons if its empty to mimik the same behaviour like in Joomla core: There you cannot edit a the User activation field if its activated once, too.
Kind regards, Bernhard