Ubuntu 18.04 won't show desktop after login

This is a known bug that's listed in the release notes. This bug affects me, please click the button at the top of the bug to say that it affects you too. The workaround is to click Cancel after entering an incorrect password and then to re-enter your password. Also, if you are able, please keep an eye on this bug and help GNOME get the logs they need to fix the issue! :)

I personally solved (or worked around) the problem by clicking on the gear on the password screen and selecting Ubuntu on Wayland instead of Ubuntu (on classic X). The bug is not present in my case with Wayland.