Specifically, the MTU value on the machine is larger than the maximum packet size you can fit through the network itself.
It manifests itself by complex or long-running queries from Notes clients, admin clients or notes designer clients failing with 'Network error: Network buffer was too small'. For instance, refreshing a mail template, or opening the admin client and not seeing all the files on the server, or a long-running agent not being able to scan databases on a server.
So you can check by using the 'ping' command, forcing (-f) the packet to a specific size )-l), and seeing if it complains. This will check host 'myhost'com' to see if it will accept 1300 byte packets:
ping -f -l 1300 myhost.com
It'll complain by saying 'Packet needs to be fragmented but DF set' if any part of the intervening network restricts packets of this size.
This might happen because you have a new machine, or you are running over a VPN that doesnt support larger packet sizes, or by running over an ADSL connection that doesnt support larger packet sizes. Or all three, in my case.
How can we validate the Windows MTU Size? Run 'netsh interface ipv4 show subinterfaces'. And you can fix it by running (in administration mode):
netsh interface ipv4 set subinterface 'Local Area Connection' mtu=1250 store=persistent
This sets my MTU on the 'Local Area Connection' interface to an MTU of 1250 bytes, and makes it a persistent change.
You don't even have to restart the machine or notes client.