Problem solved (I think!)
I noticed during yet another re-installation that while the program file was installed to C:\Program Files\, the content was installed to C:\Program Files (X86)\.
By changing the installtion path of the content to C:\Program Files\ (the same destination as the program file), the error has - so far - not come back.
The x86 is for a 32-bit version of the program, and if you have installed, and are using the 64-bit version, but install the Additional Functional Content into a folder for the 32-bit version, PrE will not be able to find it. The Functional Content is the exact same, between the two versions, but the location Path will make a difference.
Personally, I think that Adobe needs to rework the installer routine for the Additional Functional Content, to survey the installation of the program, and if it is 64-bit, then install the Additional Functional Content where THAT program will look for the material. Otherwise, I think that the installer routine should pop-up a message, directing the user exactly where to install, based on their program version. This could be a simple warning, or even better, a drop-down menu, to select the location.
Unfortunately, installer routines are usually written by another department, than the one coding for the program, or maybe even an out-of-house service, so maybe the installer writers just do not know the differences. With a two OS version of the program, with different Paths, they SHOULD be informed, and provide an easy (for the user) way to get it right. Just MHO.