Thanks for the help Rick.
It did occur to me that maybe one of the files for the index
were corrupt, so I did experiment on which file it might be. Well,
one thing I did earlier was delete the .pss and .cpd file, reopen
RoboHelp, then I could click on the index tab. I was able to auto
create a new index (small for testing), and it worked. Then, the
next time I opened RoboHelp, it didn't work again. When I generate
the file, the index works fine, but I can't change the index within
RoboHelp. I do have several backup files so hopefully some of my
'eggs' aren't cracked, but unfortunately I've had the error since
the file was created.