TCP loopback server on W5500 connects but does not receive data on the socket.

I’m using the latest Wiz driver and running the standard loopback_tcps() server ( on a WIZ850io module (with a W5500), and attempting to communicate with a tcp client ( running on a PC on the same private (home) network.

The client is successfully connecting to the server (listening on port 5000), but the first send() from the client does not trigger any receive on the WizChip (the loopback_tcps() continues to poll getSn_RX_RSR() but the returned size is always zero):

Here is a Wireshark capture showing the successful connection, but no subsequent bytes received despite the PC client having sent 1024 bytes with the posix send()

Looking up the TCP ZeroWindowProbe error, it seems to mean that the receiver (the W5500 server) has reduced its receive buffer (a.k.a. window) to zero, basically telling the sender (the PC client) to stop sending (usually for performance reasons).

Any suggestions much appreciated!

Thank you!