I have checked the server in detail. Docker is fully up, there is no issue on either disk space or server memory/RAM. The login page of central opens up fine, but it does not let me get past it. It does not even throw regular INVALID PASSWORD error if I give a wrong password. Just same old error code 500.
Just to add: The server and ODK Central was running fine since over a month, with around 2000+ submissions done from the field with no issues. Even earlier today I logged in and it was working OK.