There can possibly be no of reasons.I look at following if that helps you to start with.
If is an intermittent issue, (i.e incase where a re-run at later time works fine)
Remote host issues.
Local host agent on which AFT module is over loaded at that time(yes,I did see issues when agent is using hiugh memory).
Remote host IP(not host name) changing dynamically w.r.t their high availability configuration and you have a specific IP firewall configuration.
If standard issue,
I would make sure AFT module is in latest FixPack for that version.
Also check if any inteoperatability issues by contacting BMC supprot and FTP applicaiton vendors.
Sorry for not being specific but hope it helps to look at areas of interest.