I've created a debug version of the MultiView plugin which should help get to the bottom of this.
[ul][li]Please download the zip file attached to the end of this message.
[li]Extract the multiview.dll file from the zip to a temporary location.
[li]Exit Directory Opus.
[li]Using another program (e.g. Explorer), copy the new multiview.dll over your existing one.
The existing one is found here (if you installed to the default path):
C:\Program Files\GPSoftware\Directory Opus\Viewers
If you want to create a backup of your existing multiview.dll first you can do so, just make sure the backup is stored somewhere else and not in the Opus/Viewers folders (else Opus may still load the old version).
[li]Download Microsoft's DebugView tool and run it.
DebugView is a simple tool which will collect some messages which the debug viewer outputs. See the screenshot below.
Now try to view a file that MultiView should handle and some messages should appear in DebugView.
If viewing the file fails, open the MultiView config window (some more messages should appear in DebugView), click OK, then try to view the file again.
You can then use File -> Save As... in DebugView to save the messages to a text file. Please attach that to a reply or private message and I'll take a look at it.
The DebugView window should look something like this: