Not sure why, but this bug has been in DOpus since ages ago, and I guess nobody has reported it.
I originally found this bug following the guide here to associate RAR's/7z's/etc with DOpus, on Step 3.
When pasting text with double quotes into the Application line, I expect this...:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd go "%1" newtab=findexisting
...but I get this...:
C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd go "%1
Seems to me like DOpus isn't using the right kind of quotes in the code, or isn't escaping properly.