ERROR: Cookies are blocked or not supported by your browser

I can no longer log into my wordpress website, and neither can other users from other computers. When trying to do so I get the following message:

ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.

However, my cookies are fully enabled, and I have tried deleting cookies.

I very much doubt it is a server issue, as I have two other wordpress websites on the same webspace and I can log into these fine.

My website does not have caching software.

All was working fine yesterday, and I have not changed any plugins or themes, and I am the only admin, so doubt its plugin/theme related.

I have tried modifying the wp-login.php file to bypass the cookies check, commenting out the relevant lines as below.

// If cookies are disabled we can’t log in even with a valid user+pass
// if ( isset($_POST[‘testcookie’]) && empty($_COOKIE[TEST_COOKIE]) )
// $user = new WP_Error(‘test_cookie’, __(“ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.”));
// else
$user = wp_signon(”, $secure_cookie);

If I do this, I no longer get the error, just a white screen.

Any help would be much appreciated!