Problem solved - finally!
My Matlab 2019b and 2020a installations had several other odd behaviours in addition to the help problem. For instance none of the reports available from the Current Folder window worked. I finally decided to investigate this in more detail and found that I had a function strip.m in my Matlab path which shadowed the built-in function with the same name. It gave an error becuse it required two inputs and got only one. The error was not reported there because it was called in a try - catch section. Renaming my function to stripp.m solved both the help problem and the report problem, and some others as well.