@janvankuijk @sorgentegianluca Ok, guys. I solved the issue. It has to do with namespace case sensitivity and auto-loading. Normally, PHP namespaces are case-insensitive (see https://www.php.net/manual/en/language.namespaces.rationale.php). It seems as though, for some unknown reason, the auto-loading of classes in Joomla 4 for some PHP installations require exact letter case of the namespaced class and the file location. None of my test servers have this issue. So, I can’t diagnose it on my end. I figured it out based on examining the differences between the servers and the errors, etc.
Anyways, I got Jan’s extension up and running. I am going to patch up Version 4.0 to 4.0.1 with this minor fix, and get everyone else up and running. Thanks for the cooperation.