We’ve got a fix in progress for the assertion-on-mouse-up issue. Hopefully this will be released shortly.
I haven’t seen the other issue you reported, where the target application fails to receive the dropped file. Perhaps it’s worth double-checking whether you’re able to drag and drop the same file from the Finder into the target application.
There was a related issue recently that seemed to be caused by deleting and recreating the dropped file. I don’t see anything like that in your demo project, but perhaps this is something to keep in mind.