Specified markdown page mainpage 'mainpage.md' does not exist

Using doxygen version 1.9.5, the documenation for my C++ project builds. 
After upgrading to version 1.9.7, I get the error:

"Specified markdown page mainpage 'mainpage.md' does not exist"

Well, that file does exist, and its contents are placed into index.html 
when doxygen runs. And yes, I deleted all generated documenation before 
rerunning doxygen.

Windows 11 ARM inside a VMWare Fusion virtual machine on M1 Macbook.

I have two Windows 11 virtual machines and both do this.

I am hoping this is something obvious so that I do not have to generate 
a sample project to show this.

Thanks in advance for any help.