Automatically entering temporary guest session

Hello all,

After rebooting my PC lately, I am unable to normally connect with my user to ubuntu 16.04.
At first, I see the usual login panel. After clicking on the “username” tab in order to log in, the login panel suddenly disappears, and after a few seconds I am automatically connected as a “temporary guest session”, without any access to change anything.

I can still switch to TTY and connect to my user from there, but I would really like to have my ordinary login panel working back.

Do you have any suggestions for me?
Thank you very much in advance!