No Login to WebUI after upgrade

Hello, today I upgraded my installation via dietpi-update command. Everything worked well so far but I wasn’t able to login to the WebUI anymore after the upgrade. Everytime the message “Login failed” did appear. After countless unsuccessful login attempts I decided to make a roll-back to the previous version. Then the login worked well again. Any idea why the WebUI login didn’r work anymore after the upgrade how to overcome it?
Terminal login via SSH still worked with no problem.

Thanks and best regards.