Symptoms
Restoration of the backups created on the Plesk server and saved at external FTP fails with the error:
Global server security settings prohibit restore from unsigned or modified backups. You can not restore from it, except your service provider explicitly allows it.
Cause
The hash of the files stored in the FTP server change every time it's downloaded/uploaded: after upload and download back from FTP server with an FTP client, md5size changes.
Files uploaded to external FTP are being modified, as a result, once they are downloaded back to Plesk checksum does not match and the backup is identified as "modified".
Resolution
Contact external FTP vendor to check why the files uploaded to the remote FTP storage are being modified.
You may also use the following workaround:
Workaround
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Go to the Tools & Settings > Backup Manager > <backup_file>.
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Choose the option Restore this backup despite the fact that it does not have a valid signature and proceed with the restoration:
Note: for resellers and clients, this option can be enabled in panel.ini