The archive exists and does not work.
COMMON CAUSES
An incomplete transfer, where the download or upload was interrupted The archive was created while files were being written Disk space ran out during creation A timeout during creation, leaving a partial file
CHECKING
Compare the file size against previous backups and against what you expect.
Try extracting on your own computer rather than on the server, which gives clearer error messages.
IF IT IS CORRUPT
Try an older backup. This is why keeping several matters, and why a single backup is not a strategy.
Check whether the source still exists and take a fresh copy.
IF ALL YOUR ARCHIVES ARE CORRUPT
Something in the process is failing consistently. Common causes are running out of disk space during creation, or a timeout on a large account.
Try backing up in smaller pieces: database separately from files, or one site at a time.
PREVENTING IT
Verify archives after creation rather than assuming Keep several copies Store them somewhere other than the server that created them
