Problem Solved.
The issue was Windows Security. The Controlled Folder Access was blocking UE4.exe from accessing my project files.
In the Virus and Threat Protection Settings, I was able to grant UE4 (and other apps) access to my controlled folders.
In fact, earlier, I had tried to add a text file to the UE4 project directory, but was denied (folder not present). After a Win10 update, I was able to add the text file, but still had the original Sufficient Permission error.
Wasted a day.