Robohelp 7 WebHelp navigation pane blank

I am using RoboHelp 7 (for Word), producing WinHelp output.

In many situations, the Navigation Pane appears blank in the browser.

Here are a few situations:

Internet Explorer 11:

  - Open file on local c drive with URL

       file://localhost/B:/MoT/CIS/Doc/Help/RoboHelpProject/!SSL!/WebHelp/helpfile.htm

     - I get message:

       "Internet Explorer restricted this webpage from runing scripts or ActiveX controls"

       but the navigation pane appears and works fine.

     - If I click the "Allow blocked content" button, the navigation pane goes blank!

  - Open a web url http:// ...

     - the navigation pane is blank. 

     - adding the site to the "Trusted Sites" does not help.

If I open the web url in Chrome or Opera, I get the blank navigation pane.

If I open the web url in Firefox, the navigation pane works fine!

I guess this might be due to security enhancements added to browsers in recent years, because when I initially delivered this help system this was not a problem.

Can anyone suggest how to adjust the help file or the browser settings to fix it?