Ubuntu 18.04 on login loop, even with correct password

I had the same problem.
Then I found out that I accidentally changed the ownership of my /home/username directory to root.

So you can go to the terminal by CTRL+ALT+F1, and change the username directory ownership with

sudo chown <username>:<username> -R /home/<username>

I just spent WAAAY too much time solving this issue. What finally solved the issue was replacing the /usr/local/lib/libreadline.so.7 library with a link to /lib/x86_64-linux-gnu/libreadline.so.7

sudo mv /usr/local/lib/libreadline.so.7 /usr/local/lib/back.libreadline.so.7
sudo mv /usr/local/lib/libreadline.so   /usr/local/lib/back.libreadline.so
sudo ln -s /lib/x86_64-linux-gnu/libreadline.so.7  /usr/local/lib/libreadline.so.7
sudo ln -s /usr/local/lib/libreadline.so.7  /usr/local/lib/libreadline.so

A good indication that there was a problem was that gawk did not work because of the same missing external in libreadline with:

$ gawk
gawk: symbol lookup error: /usr/local/lib/libreadline.so.7: undefined symbol: UP

My ~/.Xauthority was fine, no amounts of updates made any difference. Right after my upgrade from to Ubuntu 18.04.1, my system would stop booting in the middle somewhere. I switched to lightdm from gdm3 with

sudo dpkg-reconfigure lightdm

and the system would boot but then I could not login.

I hope this can help somebody.

Login loop issues, type Ctrl+Alt+F3, at terminal login, if your password is correct, all is well, then at prompt type startx, then you see your desktop. The worked on two computers.