That thread made it sound like an authentication issue that day with Google?
That’s bizarre...I’ve never seen anything like that, though I don’t own a 10, so don’t know for sure there’s not some bizarre bug there. .
As others mentioned, of course reboot your device, and maybe try the gmail program?
I double checked with 12.0.1 on my iPad Air 2, and when I go to add an account and hit Google, it pops up a logging webpage immediately, like normal.