Hello Gerhard,
Oh yeah I think this is the problem for me. 
I always forget this. 
I remembered for this correctly because one of my test site the group names changed right, but the unicode_usernames setting was enabled for testing purpose. I forgot this step here.
But I didn’t enable this setting for now because I don’t want to enable it for usernames. So now I use names without accents in group names. We’ll see how it goes. 
Thank you so much for your help!
Edit: Just updated now and everything works as expected! Thanks again!