How do I get Directory Opus to show thumbnails of files?

If I open a viewer pane, I get a popup "Application blocked by Java security". "For security, applications must now meet the requirements for the High or Very High security settings, or be part of the Exception Site List, so be allowed to run."

That's coming from a viewer or component something else has installed. Nothing in Opus itself has anything to do with Java.

What type of file are you viewing?

HTML. But the error message says it's coming from WGClasses/GIFView. I don't think I've added any viewer components - I don't know how to!

Other software probably added it. In Opus, go to Settings > Preferences and then Viewer > Viewer Plugins. Select ActiveX+Preview+Office+Web, which acts as a meta-plugin for viewers written for other software. Click Configure in there and see what HTML is assigned to.

It's also possible something has messed up the Internet Explorer install. (We'll be moving to Edge in the future but the viewer pane normally uses IE at the moment.)