The backup of the domain was created and uploaded to an FTP repository;
There are intermittent connectivity issues with FTP repository: sometimes, connection fails with error from this article;
When trying to restore the domain backup from the same FTP repository on Plesk Onyx, the following message appears:
PLESK_INFO: The backup of the type cannot be restored now. To restore it, log in as the backup owner and click this backup name in Backup Manager.
Unavailability of the backup for restoration is caused by intermittent connectivity issues with FTP repository.
The incorrect information in the message is registered as a bug #PPPM-7855.
The bug has no workaround at the moment.
FTP connections issues are to be resolved on the FTP server side.