I guess my next question is - you changed your password to a hard-to-guess one - did you write it down? Is it this new password, that is giving your the "unauthorized" login? Because if you've tried the current password, and it's giving you "unauthorized", then either you've been MITM'ed (and likely, your password stolen), or simply hacked, and they changed the password on you.
IMO, Malwarebytes got it's name, by removing unwanted adware. I really am not sure that I trust it, as far as an "all clear" goes, for more sophisticated modern trojans on a system.
I would at the least, install a new HDD/ SSD, install a fresh copy of the OS from a known-clean install media (NOT created using the suspect PC), and run Hitman Pro on the suspect PC's drive, while slaved to a USB dock or something.