I thought I might save someone a bit of grief by explaining an issue I had. Recently we updated Cisco Prime Infrastructure from 3.8 to 3.9, prior to that FTP backups worked flawlessly. I noticed that 3.9 complained about backups not performing, and I thought that was odd as nothing had changed except the update to 3.9.
The FTP repository I had setup in 3.8 was using a hostname, when the upgrade occurred to 3.9 there is a default setting enabled to activate dnssec. This is documented here: https://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/infrastructure/3-9/release/notes/bk_Cisco_Prime_Infrastructure_3_9_0_Release_Notes.html
The solution was to SSH into the Prime Infrastructure Server and issue a "no ip dnssec" command at the CLI. Afterwards the backups worked as intended.
admin# conf t
admin(config)# no ip dnssec
admin# copy run start