Next time it happens, could you make some process snapshots so we can see what's blocking what?
It could be an issue with DropBox itself (we've other situations where DropBox deadlocks itself because it doesn't like too many threads making requests at once), but it could also be that Opus is doing something wrong which we might be able to fix.
Regarding the preview pane and offline files, we may add an option for that. We've always felt that if the preview pane is on it's because people want to view the files they select (else why waste the screen space, when it's a one-click toggle). But we understand some people like to leave it on all the time.