I started having this issue yesterday, it persists today. Does not affect 1.6.4 servers hosted on the same IP.
Only affects users from certain IPs. I don't believe it's a firewall issue as I am logged into SSH on that host from the IP I was attempting to connect from.
In-game and console output, 13w38c.
Removed/readded the profile, downloaded a fresh copy of 13w38c. Issue no longer present.