I can't sign into the rock account using LightDM as display manager on Ubuntu 20 Server

I’m still very new to the world of SBC’s, were my new Radxa Rock 4C Plus is my first one ever. So far, I have downloaded Ubuntu 20 Server on an SD-card, and later downloaded LightDM as a display manager.

Logging into the rock account using TTY works flawlessly, but when I try to log into the same account using the display manager, the screen turns black for a while, and after some seconds just show the exact screen as before.

The password is not incorrent, but rather works, while it feels like the display manager is struggling.

I’m using a 32 GB SD-card, and I’m shure the memory isn’t full.

Thanks for your help in advance.