Thank you Dirk.

ls -lah
-rw-rw---- 1 www-data www-data 46 Jun 3 22:08 login.json

sudo cat login.json
{"username":"admin","ownPW":"","lastLogin":""}

Adding a date in "lastLogin" did not change behaviour.

Cheerio

Thank you Volker for the quick reply. Just like johnjsb I also did a a sudo apt-get dist-upgrade before the problems started. Hence I guess dist-upgrade is a bad idea. Can anyone confirm?

Hi, I have the exact same behavior. Password for admin in GUI suddenly stopped working. I never changed the password on the sticker. Reset Password to Default does not help. The same password for user pi over ssh still works. Is there really no other fix than rolling back the to the backed up image?...