426 File Transfer Aborted Error Message

We get this message far too often, but most noticeably when the Windows XP server is unusually busy:

426 File transfer aborted [10038] Socket operation on non-socket

We get the message sometimes when trying to connect to the server, sometimes immediately upon attempting to upload/download a file, and sometimes after part of a file has been uploaded/downloaded.

I'm betting we need to change a server setting, but have no idea what the change should be.

Help!