Thanks for your report!
I recognized something else: if I try to login serveral times without any changes, I have success after three or four repetitions. Login into gnome isn’t determinitic anymore 
I found some interesting information about an unused but activated fingerprint authentification mechanism here How to disable "The account is locked due to three failed attempts" in Gnome? - this could be the answer for my problem - one of my repetitions could have been faster than the failed fingerprint. I’ll check this and report here.
EDIT
Deactivating the fingerprint sensor in BIOS seems to be the solution for my problem!