No problem with Matter. However, first Thread device will not add. It's an RPi with two SkyConnects: one running Zigbee and the other Thread. OTBR add-on seems to have started correctly. Thread credentials seem to upload to the phone (iOS) OK. But nothing actually connects ("Unable to Add Accessory: Home couldn't connect to this accessory.") I'm unsure where to start troubleshooting, as Thread has so many moving parts. It seems (clue: "Home" and not "Home Assistant") that the issue is with iPhone provisioning the device and/or Thread network to work together.
#Matter Works Fine. Thread does not.
fading palm
errant ravine
IMHO using iOS/iPhone HA Companion App to commission a Matter/Thread device has problems. It took around 5 attempts to get it to finally commission an Eve device, and for two Aqara devices it always fails even after around 10 attempts (same error message as you). I inevitably fall back to using the Matter Server itself to do the commissioning.
placid void
An important thing is that the phone that you're using for commissioning needs to be able to talk to the thread device via the thread border router. The phone needs to be able to resolve mdns records published by the thread border router, receive ipv6 router announcements sent from the thread border router, and actually have 2-way ipv6 path working for the relevant subnets.
frigid sleet
I'm running into the same problem. I added a device this morning but now am unable to add anything more and have no idea where to look for troubleshooting