Can you try and test with another project and produce a CHM output, so as to identify if it is occuring with a specific project or all of them?
Also try resetting the configuration on you Web Browser IE, as mentioned over http://support.microsoft.com/kb/308260
Could have been invoked by the Web Browser Setting?
Can you also switch you saving and generating output to the default location of RoboHelp project (in case using some other location)-- My Document > My RoboHelp Project and see if that makes any difference?
Can you share some more information (like OS, IE version, any additional Detail) case any of these does not help?
Ta 4 that. None of above helped - BUT - I've fixed it. Under 'View' I perchance clicked on View Topic. I used it to look at my Default page. It threw up a similar error message to my original. I then duplicated a page that didn't generate an error. I copied text from original Default page to new page. Used new page as default page. Deleted old page. Generated CHM with no error.
One problem remains. The Forward/back buttons don't work. However, if I run same CHM file on my iPhone using CHM+ Lite forward/back button do work.