You could post a DB bug report of failed initial backup. That might offer clues, but really a log is required. Ordinary logs are typically not detailed, but you could post whatever you can find, maybe Complete log from the job log if it got one, but often a fatal error does not, but goes into Home → About → Show log.
An Export as Command-line with suitable privacy redactions could also be posted, to get an idea of it.
Unless you’re sure you can get this to fail again, it would be best to try to preserve all the backup data as currently sitting, so maybe do a job export and use a new destination for the run to hopefully get nice log.
log-file=<path> with log-file-log-level=profiling plus setting profile-all-database queries is best but enormous and contains some personal information such as pathnames, and maybe email info, if in use.
Ideal situation would be if this were reproducible on initial backup with a small amount of data, so if it will continue to happen as you trim the backup down, that would be wonderful, but I’m not sure how possible.