Updates:
We are working on a couple of issues.
The "rendezvous" error in UD Mobile is firmware related for some, but not all, users and we hope to have a new build in the next week.
The "Could not write characteristic" error is being encountered for all users on iOS devices for Matter BLE Wi-Fi commissioning. iOS is blocking communication to Bluetooth for a characteristic (UUID) used by Matter devices. We are looking into the issue with Apple. So, currently BLE commissioning for Matter devices from an iOS device will not work, but Matter should work for IP, Thread, and previously commissioned WiFi devices.