It sounds like the reverse DNS lookup on the FTP server is failing, when it attempts to log the full DNS name of the connecting client.
See what DNS server the FTP server is configured to use...it may assist in tracking the issue down.
Possibly related is a firewall rule on the 300 that is stopping the reverse DNS lookup from being successfull.
Hope this helps.
Thank you Ross. You pointed me in the right direction.
The fix was uncommenting these two lines in my Proftpd server: