For some time now, there's been known issues with Launchpad logins and them simply 'failing' with unknown error messages.
Originally this was with registrations, and users received an error message that said "unknown error".
As of at least three months ago, this issue has evolved into logins getting the same error message.
There is no information client-side for users to resolve this, and despite multiple feature-request and bug-report posts on Meta, it remains unresolved.
This is a master thread that will become the canonical meta post on this issue for tracking