Compiling UE 4 first time - Unable to read module manifest

Getting the same error again but this time after a successful compile of the Nvidia NVRTX 5.1 Caustics branch.

On attempting to launch any of the sample projects I get a crash with the following error message "Unable to read module manifest from… Imgur: The magic of the Internet

In checking my build files I see that there has been no UnrealEditor.modules file generated.