So finally you have your application somehow working, netconn_write returns OK and you receive only 6 bytes instead of 8 and the data is wrong, right ?
Did you check the received bytes ? Are they partially wrong or completely junk ?
I wonder why the connection establishment IS traced by Wireshark and the data transfer is not. That’s strange and shouldn’t happen. Is there some wrong/unwanted filtering configured in Wireshark ? Without Wireshark you’ll probably have a hard time investigating networking issues. It’s an inevitable tool.
Is your ethernet driver known to work ?
Did you verify your setup / board with a lwIP demo/example application known to work ?
If the software is provided by ST as is you should ask them for support.
I don’t think the issue is related to FreeRTOS and also I’m not familiar with lwIP and can’t help with it.
Good luck !