Ubuntu 19.04: Xorg won’t accept any second DE

I want a second desktop environment to run alongside my Ubuntu 19.04’s GNOME. But if I install any, on the next boot the Xorg server will continuously crash until I uninstall one of the desktops.

This happens with ANY kind of DEs. Things like Unity, Vanilla GNOME,… which works perfectly fine with other people, are even not exceptions. I tried multiple times and finally ended up right here