This sounds a bit like the disease that recently afflicted me:
Directory Opus Resource Centre • View topic - Delays with PaperPort Anywhere cloud connector
Next time it happens, double click the desktop, or do whatever else you do to launch another occurrence of DOpus. Does it work? Can you get beyond the current hung window?
In my case, when this happened, I did not have to kill the whole thing, or reboot, just the currently running DOpus.exe.
The number of these events has dropped significantly since I stopped launching PaperPort Anywhere at boot.
I suspect something strange going on as the tree builds that is related to the attempts to connect to cloud stuff. I never did pin it down, even after starting to work, my way through the "General slowdown or instability investigation steps".
The puzzle for me was that even after the first hang, I could keep going with DOpus. Then again, I sometimes ended up with a set of DOpus windows that looked just like those you described, "folder tree showing contents but nothing in the file tab window". In other words, tree OK but nothing on the right.
Have you ventured into the cloud?