Backup files corrupted

I’ve just discovered that every single backup over the past 5 months is corrupted. This is over multiple plugin versions.

TarGz format. wp cron. Using Dropbox as storage. No errors at all, ever. On downloading and unpacking (mac) – error, “there was a problem while extracting the contents of the file “name of file”: Data is corrupted.
On attempting to unpack on Dropbox, same error.
Tried manual run backing up to email but the file is too large to be created out of memory error.
Tried manual run backing up to a folder. No errors at all but the file was never created. Tried a different destination folder. No file created there either.
Tried manual run to Dropbox. No errors. Same file corrupted problem.

Suggestions?

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