This is an issue with the FTP servers that happens at times due to maintenance. The way the FTP servers work is they are in a cluster and when they undergo maintenance, some servers are taken out of the cluster meaning more demand is put onto the other servers. This results in the servers sometimes being overloaded and some users getting this error when attempting to connect to FTP.
This issue should usually fix itself within a short period of time. Simply try again about 10 minutes later and the issue should be resolved.