Javascript error when running Microsoft HTML project

Hi all

I'm using Robohelp 2019 Classic, version  and am trying to generate a Microsoft HTML CHM help file.

It seems to generate fine, but when I launch the CHM file I get the following error:

I have tried:

• Updating DHTML
• Editing the ehlpdhtm.js file (which seems pointless as it is re-generated each time)
• Clearing the error, and clicking another topic. The same error re-appears.

This happens with all my Robohelp projects, but only for CHM  output. Other output is fine.

Can anyone help please? 

Thanks

Laura