I randomly found this answer to another question that worked for me, too. To summarize the yunohost backup interface for some reason (when the corresponding option is enabled) creates the bakups using the .tar.gz extension keeping the file a tar archive. This mismatch between the extension and the file creates the problem.
Solution: rename the file from yunohost-vps-mastodon-pre-upgrade1.tar.gz to yunohost-vps-mastodon-pre-upgrade1.tar