If this has only suddenly started happening, it suggests something changed with the server setup, such as the temporary directory (usually /var/tmp) is full.
13 days ago
The error message means the form token is not equal to the token saved on the server (in the server session).
If you still have access to the AdminCP, can you try enabling debugging in the AdminCP -> Core -> Debugging and Maintenance tab? Then have a look at the login page and see if any errors appear right at the top.