Mark Smith:
The "failed: 4" tells us that the error was CRC_ERROR (see https://gitweb.torproject.org/tor-browser.git/tree/toolkit/mozapps/update/co...).
The update.log file should contain more details, but that may be gone now. You might look around for a file named update.log, backup-update.log, or last-update.log
Seems I still had a backup-update.log lying around, yay. It is attached.
Georg