im having the same problem, except in my case as a developer on AMO, it means i can no longer update my extensions etc. whatever change has been made on AMO has not been communicated very well, to suddenly not be able to login like that is quite a shock.
i did see a message saying they were migrating to a new login system on the login page for quite a while, though when i attempted the migration process it didnt work for me, so i just kept on using the same old login system. If the old login system has now been turned off, there should at least be some kind of error message saying, “you need to migrate your login now” rather than just saying “unknown account”, its all rather jarring. I hope this can be rectified soon.
I also noticed that there was a period when logins were disabled on AMO yesterday, due to server maintenance, i guess the maintenance has either introduced a bug where the old login system has been switched off, just a general bug where all logins are broken, or just a badly thought out migration process.