Plesk backup cannot be created, restored, or downloaded correctly: Can't find public key in database OR SignError Can't initialize RSA private key

Symptoms

• Creating Plesk backup or doing the migration finishes with a warning:

  Warning: Unable to upload the backup to the FTP storage: Unable to set backup sign certificate

• Restoring a backup in Plesk fails:

  Error: backup_sign: Can't find public key in database:
  backup_sign: Can't get server's public key

• The following error occurs during the creation of a scheduled backup in Plesk:

  Unable to create remote backup. Error: Unable to export backup: Can't initialize RSA private key

• Backups downloaded from Plesk are empty or very small in size, and the following error is found in the log file /var/log/plesk/PMM/migration.log or %plesk_dir%PMM\logs\migration.log:

  INFO: SignError Cannot read public key [common/plesk-utils/PMM/backup_sign_lib/backup_sign.cpp:SignError]
  void plesk::composePublickey(const string&, std::__cxx11::string&, std::__cxx11::string&)

  ---

  INFO: SignError Can't initialize RSA private key [common/plesk-utils/PMM/backup_sign_lib/backup_sign.cpp:SignError]

• Migration pre-check shows the following warning:

  stderr: rsync: link_stat "/root/plesk_migrator/plesk_migrator-9xcxzh5oqyrrqyfdhjs8dezlk536xa5p/backup.tar" failed: No such file or directory (2)
  rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1650) [Receiver=3.1.2]
  rsync: [Receiver] write error: Broken pipe (32)

  ---

  Failed to merge the configuration backup of subscription 'example.com'. Not all subscription settings may be migrated.
  Error message: syntax error: line 1, column 0

Cause

Backup sign certificate is missing or corrupted.

Resolution

Note: The solution is intended for use only by server administrators. In case there is no administrative access to Plesk, contact the server administrator or server provider.