An unknown error occurred while signing into apple id
Yesterday my mom got a prompt to sign in to her apple id when she tried to send a text through Messages. It gave the "An unknown error occurred while signing into apple id." I have never had this happen before with any Mac I've owned in my life (and we've had them since the 90s). I immediately restarted and also tried shutting off wi-fi and turning it back on since I read it can sometimes be an Internet connection issue
I proceeded to scour the Internet for fixes. I've tried nearly everything including deleting the related keychain files and preference files associated with it. I restarted each time after doing so. No dice. I created a new admin account and tried to sign in there, nope. I tried signing into the new account using MY Apple ID and it gave me the same error. I tried restoring from a previous time machine backup to the last time she remember successfully sent a message using her Mac. She is running Little Snitch, but I tried disabling that as well, and she still can't login.
What's especially strange here, is her Apple ID is fully functional on her iPad Pro and iPhone 6S. She can send messages and everything is working fine there. It's just her 2015 Macbook Pro that is completely shutting us out of Apple ID. It won't even let me sign out. It just hangs and does nothing.
She is still running Sierra. I never updated her, because I'd have to migrate a lot of her Adobe CC apps and actions and plugins. And a lot of that stuff breaks with new Apple OSs. It's the only thing I can think of that maybe it just needs a newer OS and something finally tripped it up.
The last thing I'm trying now is just doing a re-install of the OS to see if that works. Otherwise I'm absolutely stumped on this one. I read it could be some sort of spam block enabled server-side by Apple, but wouldn't that also lock her out on her iPad and iPhone?
Another side note, we don't have two-factor enabled on anything. I realize it provides extra security, but if anything happens to our phones, we don't want to be locked out of our Apple ID. Hence we never enable it, and Apple almost monthly logs us out of our Apple IDs and makes us unlock our accounts. I know it's an effort to push two-factor on us, but we really don't want it.
Any Ideas?