Hello René,
Oh wow, I'm so sorry for this - I totaly overlooked this thread because it was marked as "answered".
On my VM, I had a really strange error "Failed to start .NET" - so not really the same as yours. This ended up happening because I did not have the DLL file on my VM and I was building the project using dummy resources.
After fixing that, everything worked just fine on that VM as well as on my own machine.

- Screenshot_232.png (312.9 KiB) Viewed 14917 times
After further playing with this, I think the issue actually is the fact that you do not have the WindowsForms support on your end.

- Screenshot_236.png (5.08 KiB) Viewed 14916 times
On my end, when I had only the runtime installed, I managed to reproduce the same issue:

- Screenshot_233.png (176.11 KiB) Viewed 14917 times
Please note that in order to have the WinForms support, you need to .NET
Desktop Runtime and, of course, a version matching yours from the project, aka 8.x:

- Screenshot_234.png (34.87 KiB) Viewed 14917 times
Here is a download link:
https://dotnet.microsoft.com/en-us/download/dotnet/8.0
and you can download the SDK as it contains all the others:

- Screenshot_235.png (47.68 KiB) Viewed 14917 times
Hope this helps!
Best regards,
Catalin