Can you reinstall the firmware/OS?
As I understand it you can download the software from bitmains website as it looks like your login data may have become corrupted or something.
To clear something up, the passwords for the GUI will be stored in a different place than the SSH/server ones for security reasons along with mere simplicity (if I understand it correctly, Linux is the backbone of the antminer OS).
As far as I am aware, you can only upgrade/reinstall firmware from the GUI which I cannot access.