I see that this has happened multiple times in the past. I recently updated to latest beta, and this error occurs regardless of vehicle type or dev/stable selection, even after deleting the sitl folder.
I’m running build 1.3.8321.15811 under Windows 11.
I can connect via UDP/TCP if I run SITL from the command line in a WSL build environment.
Strangely, everything is working fine for me this morning without changing anything. A reboot might’ve helped, but I had tried that already before creating the topic.