Think that all you've done, including hitting abort, to make the situation unrecoverable. Sometimes a task gets hung up at the finish, just before or during preparation of the upload file, phase 1 of the reporting. A client restart may have kicked things back in motion.

Eventually BOINC will run a cleanup to get rid of the entry, not advising to go edit control files. What you also could do is run the buffer empty, stop new work fetching, then do a project reset. Expect that too to clear the now bad entry.

I can assure you it wasn't the abort that caused the problem.
I tried abort as a last resort. The PC had been sitting for many days with this task in upload state beforehand, I had rebooted it several times. I had also updated the boinc client - and rebooted a number of times afterwards before I finally tried abort to get rid of the task.
This particular machine has been running boinc for many years without any issues.

I have one sitting on machine for over a year now. If I were you I would just leave it alone and let the machine go on crunching.
Cheers