Ubuntu 18.04 Login window loop

Is there any reason the login window would stop working and start looping? I'm running 18.04. I normally RDP into the ubuntu box, but that isn't working now. Using a keyboard and mouse, I see my username on the monitor. I type in my password. Then the screen goes blank for a few seconds... and I'm back to the login screen where my username is. I'm not getting through to my desktop!

I haven't messed with the machine except for the following, but, I'm not sure why any of these would impact the loginscreen.

• install pihole in a docker container,
• turn off systemd-resolved (to get port 53 working) per https://www.reddit.com/r/pihole/comments/8zamlk/trying_to_install_pihole_on_ubuntu_and_having/e2hd7qp/ and How to disable systemd-resolved in Ubuntu?, and
• installed gufw to graphically manage the firewall/open port 80 for pihole.

But none of these messed with the login manager. Does anyone have any idea what could be up? Interestingly, nearly all of my docker containers (portainer, heimdall, tatulli, sonarr, radarr, pihole) work, with the exception of anything using ssl (plex and cockpit). I'm guessing somehow the firewall blocked port 443, but I'm not sure why that would stop me from logging into the box using a keyboard and mouse...

Any help/insight is greatly appreciated!

EDIT1: I tried creating another user via

sudo useradd -d /home/testuser testuser  
sudo passwd testuser

but the login manager still throws me for a loop

EDIT2: I've disabled the firewall via

sudo ufw disable

and I reversed the system-resolved.service work, but still no luck :(

I've almost done all suggestions here, but none of them didn't work. Also gd3 reconfigure has no effect. But I had suspected to my last gnome extensions.

The [email protected] was the problem. So I disabled all gnome extensions to solve login loop as following steps:

1. Press Ctrl+Alt+F2 or Ctrl+Alt+F3 or ...
2. Enter your username and password
3. cd ~/.local/share/gnome-shell/
4. mv extensions extensions.bak
5. mkdir extensions
6. Press Ctrl+Alt+F1
7. Login and Thanks God

Hope help somebody else. I almost worked full day to find this solution.

PS: I've installed my favorite extentions again and I have to tell you Login Loop disaster in ubuntu has wide variety causes. So try to find your special cause and solve it.

Looks like it may have been related to "upgrading" from 18.04 to 18.04.01: No login screen after ubuntu 18.04.1 update

In any case, doing the following fixed it for me:

systemctl stop gdm.service  
systemctl restart gdm.service  
sudo dpkg-reconfigure gdm3  
reboot