Looks like this is a UE4 engine bug in 4.25.1 - breaking support for Windows 7 :(
https://answers.unrealengine.com/questions/964896/view.html
I don't have time to build the engine from source at the moment, but I will update the build when 4.25.2 is released.
Only fix right now is to update to Window 8.1 or above I think.
Sorry!!