Try to remove bg3 from windows firewall.
I just double checked and I have exceptions for all the .EXE's in the Firewall. Also turned off Avast and still no changes. Game will not start.
I too have a similar issue as the OP - tried switching to DX11 and back and the game now refuses to start or still tries to start in DX11 mode. However, i come to think it's a problem with the launcher because when i started the bg3.exe in [...]\steamapps\common\Baldurs Gate 3\bin, the game ran perfectly fine.
When I try to run it from the .EXE it will begin to run but then the launcher will suddenly open and the game will close.