is this machine behind the same router that the working machine is behind?
Yes - it is.
I tried the Docker container, same result. The machines are connected to a switch (no network management configured/hub mode), then NAT router → internet → NAT Router → WiFi → Win 10 (I do remote desktop support for my 90 year old mom!). The working Win 10 PC is connected to the next port on the switch (i have swapped the cables around as well). I must be running this beautiful ZT network and Nomachine setup flawlessly for 2 years, just never tried ZeroTier on a Linux machine.
MTU on the zt interface should be 2800.
That was my doing. I tested the 1500 MTU limitation, had a Cisco config issue once. Will change it back. Yes, Ubuntu 22.04 has Network Manager in place of Systemd-networkd , and I must admit it is still dark arts for me.
I used the command ifconfig zteb4mbztl mtu 1500 up will change it back, forgot about it.
I downloaded the ZeroTier source code from GitHub and built it. Same result as the package installs. I wish I had the skills to read the code.