Can no longer login to sites on 1 server, no problems on another server

Hi,

I can no longer login to any WP site hosted on our dedicated server (ssl secured). It returns the login page with the username/pw boxes emptied.
Yesterday all was still OK and I changed nothing anywhere + there’s no problem on WP sites hosted at another (shared) server (no ssl).

Example of the url I get after login attempt

https://mysite.com/wp-login.php?redirect_to=https%3A%2F%2Fmysite.com%2Fwp-admin%2F&reauth=1

All the sites on both hosting accounts have the same theme and plugins so that can’t be the problem.

Help?

Thank you.