@Kent-Dorfman the error gives the SAME file - the moc file. What is strange is clean builds on Windows works (my copy using Beyond Compare leaves out the .user and build files) but not clean builds on Fedora.
The CMake environment was done using QtCreator. And as I said, I deleted the build directory on Fedora and the problem still occurs. Still trying to get my Ubuntu environment to work (CMake can't fine Qt; I've fixed that before on older systems, but did not have time tonight)
So far, it seems to be something due to Fedora or Linux. I did reinstall Qt from qt.io instead of using what fedora wanted to install. Same problem. This makes me wonder if it is due to differences between MingW (on Windows) and g++ on Fedora.
If I try CMake directly on Fedora, it can't find Qt. Once I fix that on Ubuntu, I will see if I can fix it on Fedora.
Thank you for your answer, and I will get more information