Just wanted to throw this out there for Linux users - I had this same issue and wasn’t able to open any projects. I’d built from source on the 4.16.3 branch. When saying “OK” and trying to rebuild the library, i found this in the stdout output;
fatal error: file '/usr/include/linux/version.h' has been modified since the precompiled header '/home/<username>/projects/ue4/UnrealEngine/Engine/Intermediate/Build/Linux/B4D820EA/UE4Editor/Development/Engine/SharedPCH.Engine.h.gch' was built
note: please rebuild precompiled header
I had recently updated my linux kernel version, and this included the kernel source specified above (the /usr/include/linux/version.h). The solution was to rebuild the editor ( make configure; make ) and then try to reopen the project.