Now, the hubs cannot be full duplex. Full duplex requires double-buffering on each port. The switch is probably full-duplex, but it may or may not properly fall back to half duplex operation with other equipment. Your server sounds like it's doing full duplex, and the hub or switch it's connected to is doing half. When the server is recieving, it thinks it's OK to send.
Most hubs only connect at half duplex.
But now a key server is showing me some errors.