Can't sign in to icloud

I was randomly signed out of iCloud on my Mac Pro (10.13.6), and now can no longer sign in for Messages, Notes, etc. It was working fine for years before this. I have reset Apple ID password, checked all connections, signed in online and removed the computer from devices list and re-added. I've also already tried phone support, they had no answer and actually told me to "go to the forums."