Ok I couldn't get it to start through the GOG launcher. So I launched it from the bin folder and things went ok... a bit buggy here and there... I played all the way to 3rd level with no long rest.
When I went to long rest, I hangs up where the cut-scene dream sequence starts. I've tried all the things listed here...

...is the perfect way to persuade people not to make helpful suggestions.

mystery solved

install the net 6.12 and launch it from the correct Larian launcher i.e, as designed

Thanks for your replies-
I got it on stream and it works right out the gate.

Both Steam and Gog servers now give you a different Larian launcher that doesn't block play if you don't have the lastest net installed

I find it old that you came here to fix a Steam issue but glad you have it running now