I have found that some of the file types i.e. HTML will not open at all when clicked. I have to drag them into the browser for them to open. But if I do it in explorer it work as expected. Now TXT files open fine. Not sure why this would be.
Please go to Settings > Default Apps in Windows and change the default web browser to another one, then back to the one you were using.
That will most likely repair the registry settings and make things work again.
If it still does not work, please go to Settings > File Types in Opus, then use File > Diagnostic to create a diagnostic file of the HTML file type, and attach that to this thread.
Opus_FTD_File_html.zip (42.0 KB)
The registry looks like it should be opening HTML files in IE.
Try right-clicking an HTML file and using Open With to change the default app. That may fix things if the other method didn't.
If it still doesn't work, it's possible antivirus/firewall or other security software is blocking dopus.exe from launching IE.