I got this issue (UiPath.OutOfProcessUi.ServerHost.exe) when trying to use Forms activities(v.1.9.6) and I just followed this topic with no solution.
I’ve found UiPAth redirects you to .Net runtime v.5 and finally I’ve solved by installing the whole .Net v.5 version instead.
To use Forms Activities with .net5 i.e. v1.9.6, you need to have .NET Desktop Runtime 5.0 (x64) installed. Kindly let me know if you have any query or if you are facing any issue.
As said, It was not only enough the fact of installing the runtime,
I had to uninstall .NET v.4 (and the runtime from v.5) and install the whole bundle I mentioned.
It’s just another solution