same issue since yesterday, sidecar was working for few days, but has stopped with this error message. restarting both devices, no effect.
I was having the same issue the other day. The USBC-USBC cable I was using momentarily loses connection on occasion, so I then tried it on Wi-Fi and was able to get it to work. If you are using Wi-Fi for your internet connection for your Mac, either turn off Wi-Fi on your iPad or make sure they are on the same network.
Hope this helps.
(Note: This was on a 2015 MacBook Pro)
Electricespeon, I was also experiencing this issue. I have found that it may have something to do with being plugged in at the start of sidcar. When I unplug my iPad Pro 3rd Gen (Thunderbolt) from my MacBook and then start sidecar, its almost instant and I can use it as a second monitor. Then I can plug it back in so that it can charge while I use it as a second monitor.
If it's plugged into my mac at the time of starting sidecar...both screens will flicker black while the ipad remains black and even has the sidecar touch controls; ultimately it closes on the ipad and i get the Error 401 on MacOS. Unplug the ipad, and works instantly again.
Give this a go. Let me know if this helps. Thanks
I have tried this and get the same error either way, but after updating to beta 2 on all my devices its giving me new errors "Unable to retrieve NWConnection for ClientID" when I try it with no cable and "GKVoiceChatServiceErrorDomain error 32002" when I do it with a cable. Quite strange
I get the same error and it started after update to beta 2.
The error happens using cable or wifi.
"Unable to retrieve NWConnection for clientID"
Apr 23, 2020, and still hasn't been fixed yet ~ SideCar is still in the ditch.