You must use the "set" method to update an entry. (Again)

Shouldn't $allowedFields be an array?

PHP Code:

protected $allowedFields = ['role_id', 'first_name', 'last_name', 'username', 'password', 
    'email_address', 'mail_me', 'ack_round', 'language', 'referee', 'xp', 'uuid', 
    'activation_token', 'activation', 'dsgvo', 'notified']; 


And in your case, it is an array of one string element.