I am having a similar issue:
A scene that works perfectly in ue 5.0.3 can’t even load on 5.1
I have tried all of your advices up there, like:
-disabling SM6 and use SM5
-Trying DX11 or DX12
-Using developer nvidia driver
-Repairing Ue5.1 installation/uninstall and reinstall
-enable/disable Lumen global illumination and reflection
Nothing worked for me, I am having a memory error in this case about Ram cause it says the paging file is too small, I have tried to add more memory there but anything worked for me I spent 24 hours almost trying to get this working cause I think the foliage with nanite of 5.1 is needed.
My pc specs:
CPU: AMD Ryzen 7950X
GPU: RTX 4090
RAM: 128GB 6000Mhz
I mean I was able to use it in 5.0.3 without problem and with a very good fps now in 5.1 I can’t even load the Scene. I don’t think honestly is something about my pc specs I mean I am using very overpowered machine.
I went back to 5.0.3 I hope they will do something cause by reading around lot of people are experiencing weird stuff. If anyone can give me any other advice it’ s apreciated I will try and let you know