DavidGalloway and matfer, I had a similar problem, though I am running on Linux. I think the error message that the editor displayed in a pop up was similar to what you have both written, but the message was not saved in the logs as such.
Here’s the what I did find in the logs:
LogLinux:Warning: dlopen failed: /path/to/project/Binaries/Linux/libUE4Editor-project.so: symbol _ZThn1064_NK5APawn24GetNavAgentPropertiesRefEv, version UE4 not defined in file libUE4Editor-Engine.so with link time reference
LogModuleManager:Warning: ModuleManager: Unable to load module ‘/path/to/project/Binaries/Linux/libUE4Editor-project.so’ because the file couldn’t be loaded by the OS
I resolved it by renaming /path/to/project/Binaries/Linux/libUE4Editor-project.so (I did not want to delete it in case it was the wrong thing to do) and restarted the editor for the project in question. The editor asked me if I wanted to build the missing .so which I let it do and that fixed it.
If this is the same problem you are seeing, I expect you’ll find the relevant .dll under /path/to/project/Binaries/Win64/libUE4Editor-project.dll. Try renaming that and restarting the editor (probably best to not be in the editor when you rename that dll).