Reinstalling/repairing Word may fix the registry entries (there's may be something broken in them which is tripping up Opus but which Explorer doesn't depend on).
Antivirus could also be blocking things, e.g. stopping Opus from launching Word if it isn't already running, but allowing the action if it is already running.
If you link your account we can also look at:
- Registry dumps saved via Settings > File Types > File > Diagnostic in Opus.
- Process Monitor logs of what happens when you double-click one of the files.