Hi Jürgen,
you made my day! That’s it!
I would have never expected that. 
I tried different combinations now and 8 characters of letters work. As soon as I use numbers or specials it does’nt work again. So I’m really thankful for your suggestion.
May I ask if it will be possible to use more complex passwords in the future?
For now, I’m happy that I got it up running. 
But a little bit more security would be nice.