Updated to 26.1 can't sign in to Messages/Facetime any more

I made the big mistake of updating to 26.1 yesterday and since then, I have not been able to sign in on FaceTime or Messages.

I am signed in on my appleID, and photos, iCloud, passwords, notes etc are all working fine.

What I have tried:

• signing out of my appleID, rebooting then signing back in again
• checked all the proxies are off - as I read on another thread that this was causing an issue for some (they were all off)
• connecting through a different network and through VPN

Additionally, I have identified that I can no longer screen mirror from this device to anything except my AppleTV - I used to be able to mirror to other Macs in the house - and no device is able to mirror to this device that is having the problem.

(This is on a MacBook Air M2)