We have found other reasons for this after some poking around:
- Windows firewall can in rare cases block the connection
- Router can do the same
- Bad routing? contact isp and ask how your being routed (this is just something i have heard about atm)
- Anti Virus blocking.
but no matter what it seem to be releated to the clients network in some way
hope some of that helped :s
In my experience it has to do with a player dying and being disconnected around the same time.
Players experiencing this can connect to other servers with no issues. I haven't heard of this as an issue where someone wasn't already playing on the server previously.