I have discovered another cause/solution to this issue. I was using a cloud share (MS OneDrive) for my web files (this is small dev environment at home). I created a new web dev environment on a new laptop, and I hadn't yet set my web files to "always keep on this device." As a result, when the php engine went to fetch a file, the file really wasn't there, even though I could see it in File Explorer. Once I forced all my files to reside on the new device, it worked like a charm.