I was moving a folder from one drive to another. While that was going on, I tried to shift delete a 0 size file on the source drive. Then DOpus froze, and I couldn't even kill it via the task manager. Windows wouldn't shut down either, so I had to use the reset button. I think this might have been one of those confluence of events that happens rarely. But I mention it never the less, in case it's of interest to the developers.
If Task Manager can't kill the process, it means an API call is still waiting at the kernel level and hasn't returned, which indicates the problem is at a very low level and outside of Opus itself. (Hardware, drivers filesystem, antirvirus or the OS itself, generally.)
Interesting. That explains a lot. Thanks for the info!