@Aury88 sorry, I didn't read the whole thread, just from the point you pinged me.
That log isn't related to https://github.com/MirServer/mir/issues/704 where one of the DRM functions returns a misleading result. Instead it looks like the graphics driver isn't adequately supported by Mesa.
I know that my colleagues at Canonical have yet to get graphics working on the Pi4 with Ubuntu Core (I think that's lack of time, not a technical issue). But maybe you can find something useful here: https://forum.snapcraft.io/t/support-for-raspberry-pi-4/11970
you might need to play with the dtoverlay= kms or fkms values in config.txt. i know the vc4 driver isnt fully ready to drive the vc5 hardware in the pi4 but i know that for example my omxplayer-pi snap works fine (only 1080p though, omxplayer does not support h265 yet) when either using no dtoverlay or with the fkms one (not with the kms one though). - https://forum.snapcraft.io/t/support-for-raspberry-pi-4/11970/17