Three years ago I wrote a Band Decoder freeware app that communicates using TCP packets between a Windows GUI console and a Raspberry Pi CLI controller. Has been working fine here and at a few dozen QTHs around the world. I recently designed a miniature RPi HAT board with 16 opto-MOSFET "relay" outputs, replacing the original design of 2 x 8 SainSmart relay boards.
The HAT board works on both Pi-3 and Pi-4 hardware, but on an Pi-4_8gb (my only model 4) the Pi Controller software receives an initial TCP packet from the Console and thereafter refuses to accept additional packets with an "Error on accept" at this line of C++ code:
newsockfd = accept(localSocket, (struct sockaddr *) &cliaddr, &clilen);
Any advice or suggestions on further troubleshooting would be appreciated.