I have a login screen loop back problem, when I punch my password I get back to the login screen again. I have tried to login with LightDM CI screen by Ctrl+Alt+F3 but there when i put in the username and password it returns incorrect login.I want to try the options of Ubuntu gets stuck in a login loop but to run the commands I have to login. This happened after a repair of Grub.
I have encountered the same problem with you and now I have solved it. Maybe my solution helps you. You have mentioned that this issue happened after a repair of Grub, so open the apt log file (var/log/apt/history.log) and check the last time when it worked, then you can revert(remove or reinstall) the package that you changed the last time. Then reboot, hope it works for you.