Facetime in Mavericks - permanently broken?

I've used Facetime daily in Mavericks until last week. I had to reboot after a Bluetooth issue, which included deleting the Bluetooth plist. On restart, trying to use FT I was asked to sign in, which I'm NEVER asked normally. On entering my Apple ID plus password followed by 2FA 6 digits, I was met with the message 'The server encountered an error processing registration', and that's been the situation ever since.

I also have a High Sierra VM where FT works, but the person I'm FT'ing hears an echo of their own voice which may be due to it running in a VM? (It wasn't there when FT'ing in Mavericks).

Trying to restore FT files that had changed after the reboot with their equivalents from before, using Time Machine, didn't cure the problem.

Tech: 2011 i7 iMac, 16GB RAM, 1TB SSD, Ethernet internet, OS Mavericks 10.9.5, VM OS High Sierra 10.13.6