Sigh. Are you using the same hostname? (Maybe so?)
That’s your problem now.
Did you configure S3 in the database or in the yml file? I guess it’s the former. The easiest thing would be to restore a database-only, but that’s hard to explain (extract the sql file and then name it with the same stuff as the tar.gz file)
You can try this
discourse restore --pause <backup_filename>
That will pause before it tries to restore the S3 stuff. Then, if you knew what to do, you could fix the issue with the errant uploads.
See Issues with restoring backup to discourse - #16 by gerhard
Another thing would be to set DISCOURSE_USE_S3 to false in your app.yml. You might need to set the other S3 stuff too like the ones that say to use S3 uploads and/or the S3 buckets.