[2021] [Linux] CK3 works, but launcher is black rectangle

Almost certainly this issue, I bet:

[Launcher 2021.1 Linux] Launcher interface appears completely black when used with NVIDIA Prime Render Offload

My launcher just updated, and the launcher interface appears completely black. It is still functional, but you just can't see everything. You can launch the game, close the launcher etc. just fine if you know where the buttons are. But it is... forum.paradoxplaza.com
If you need help specific to your distro that thread would be the best place.

All the best!