We have discovered an issue with firmware 3.4.3 on a Duet Wifi. The symptom is that when FTP and Telnet are both enabled using M586, the network interface stops working and DWC is unable to connect.
I have identified the reason and posted a temporary Duet 2 firmware binary to fix it in this thread: https://forum.duet3d.com/post/296505.
I was unable to reproduce this same issue on a Duet 3 Mini WiFi, however this issue might affect that board too. I will release new firmware binaries for Duet 2 and Duet 3 Mini next week.
Boards using Ethernet interface are not affected by this issue.