Login loop after upgrading to 19.04

I had to disable the Extensions Gnome extension.

1. Being at Login screen go to the second terminal (CTRL+ALT+F2)
2. Login as your user there
3. Execute gnome-shell-extension-tool -d [email protected]
4. Restart the system or go back to the first terminal (CTRL+ALT+F1) with GUI Login screen and try to login again.

Please look at /var/log/syslog and check if you see some Gnome crash stacktrace.

(As @robgrune said in comments, 19.04 seems to be particularly susceptible to extensions, and @Milso's answer is on the right track, but in my case creating a new user didn't fix the problem.)

If you create a new user and still can't log in, then it's possible that a gnome extension package is causing your problem.

In my case, I had to uninstall gnome-clocks.

sudo apt remove gnome-clocks

There are also other packages that are actually gnome extensions but, unfortunately, not all are listed as extensions. To see all the gnome packages chosen:

sudo apt list --installed | grep gnome | grep -v automatic

Then uninstall one-by-one until you can log in again. You can always reinstall them again afterwards.

One possible cause for a login loop is installation of chrome remote desktop in ubuntu 19. Remove it via the terminal, and login works again.

sudo apt remove chrome-remote-desktop