Running 18.04 from a USB key, everything is fine. Then I tried 2 installations. For the first time, I checked "install third-party software". the login screen reappears after the password is entered – at the command line, I find that startx fails with an unspecified abandon of x server. Second time, I did not check "install third party software". Now, the login screen will not recognize my password, so I can not use the command line to investigate further. Yes, I made sure of the passwords :]
This is a new PC with Intel processor and Z390 chipset with Nvidia RTX 2070 gfx card. This is dual boot with Windows 10.