TCP spurious retransmission


I use a W5100S over SPI with ioLibrary_Driver.
Configuration is done via DHCP and works fine

when testing my application with putty and wireshark I experience lot’s of
TCP retransmissions in both directions . These are send despite an ACK being sent already.

After approx 40s the connection times out and is resetted.

What could cause this problem?

Best Regards