Hello,
The way it worked for me (Case was an Unreal Engine 4 Project and I wanted to open it in 5.1 ) was by deleting the (Binaries + Intermediate) Folders → Right Click on the project (.uproject) and generate Visual Studio Files → then I opened the project .sln file which is a Visual Studio file included in the main folder of the project → manual re-build from there and checking for errors (errors I encountered were mostly the #include directories which refers to the old engine version and the my old machine directory) 
Note: I am not a developer but this how I fixed this error for the case I mentioned earlier.
Hope this helps
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