These forums are locked and archived, but all topics have been migrated to the new forum. You can search for this topic on the new forum: Search for "Backup failed : FTP server hostname cannot be resolved" remote ftp backup stopped working... on the new forum.
PS A server reboot fixed the issue. The server was hacked two weeks ago, and a cryptominter was installed. I forgot to reboot after cleaning it up (deleted some stuff in etc tmp) :)