I’m sure Jeff was just following protocol - maybe he had to ask questions about my system he thought most people wouldn’t want to share publicly.
Anyway, for others’ future reference:
Jeff correctly pointed me to the help files’ location, where I could confirm they are indeed installed. And where I discovered that they are PDF files. Combined with G-String’s advice about requiring a functioning Acrobat Reader - it became clear: never mind a corrupt pdf reader - I know I have NO pdf reader installed. Don’t recall seeing a mention of it being required.
So I downloaded an offline Acrobat Reader installer from Adobe, transferred it to my production machine via USB key, instaled the reader, re-started Cubase - - and ta da - - help files open like a charm. I’m guessing the Documentation buttons are just pointers to the pdf files, which depend on system asssociations to call the required reader.
So - thanks Jeff & G-string.
And yet another revision item for future for the programmers - perhaps an error message for the case that a pdf reader wasn’t found, not that the files weren’t found?