I have been conversing with support today.
It looks like my problem may be related to a race condition, where I downloaded the file in question on to my Zonbu before I fixed the caching problem, thereby causing a timeout on upload.
Apparently, and to my surprise, it does eventually give up, silently. I indicated it would be good to let the user know if uploads fail, and prompt to retry.
I’ll confirm the situation when I get home tonight and post here accordingly.
I am told that renaming the file should kick off a retry on the upload.
This seems like an area that needs a bit more vetting to survive the average end user and broadband imperfections.