The problem is solved.
It was due to, shell we say, a human factor.
>The above should be enough... just to make sure that there aren't any oddities with the path (macros with spaces or other funny characters, double or missing backslashes), can you try this:
>
>
c_helpdir=forcepath("help",c_homeDir)
>x_r1=fullpath(forcepath("usbu.chm", c_helpdir))
>set help to (x_r1)
>
A moment of silence is our cosmic reset button.