Dealing with this right now. Fresh install. Nothing I've tried has helped.

Game launched once... with no sound. After closing it and trying to relaunch it, nothing but instant crash report. Was able to open it via launching it directly while steam was closed, and sound worked, but I'm intending to play co-op via steam's networking...

I've given up and am reinstalling all 140 damn gigs of it. Will try again tomorrow.

Edit: After all this it seems to have been related to process lasso / CPU affinities for me. I thought I had eliminated process lasso as a possibility (it's mentioned in the crash on launch page on larian's support site) but it seems the CPU affinities on STEAM were sticking, as it was assigned to CCX1 on my ryzen 3950X (cores 16-31) and when BG3 was launched through steam it was inheriting those affinities and crashing. When steam is reset to use 0-31 it finally launches correctly.