Try the IStream registry file here:
[Drag & Drop does not work for files or when customizing toolbars)
It's not the exact same problem but the registry file can't do any harm (at least on current versions of Windows) and it's solved some other drag & drop problems for people in the past.
Restart Opus after running the .reg file, just in case.
If that doesn't help, have you modified the Drop event on any of your filetypes?
If neither of those are the answer then I'm not sure what the problem might be. Best to contact GPSoft directly for support.