Why do my sockets not work after upgrade from fedora 30 32 bit to fedora 31 64 bit

I’m stumped.

My C++ software uses QTcpSocket etc and has run flawlessly for a decade.

On my new fedora 31 installation the sockets appear to open and write correctly but are not seen at the listen port. ss shows the port is open for listen on any address.

The differences between working/non-working systems are:

(1) the old systems are 32 bit fedora 30
(2) the new system is 64 bit fedora 31

(3) the 64 bit system is being installed on a newly purchased 7010 Dell 7010 system

Any clues as to what I should do? I’m at a complete loss.