Just got a reply from the hosting company:
No, no php problem. We routinely shut off permissions to wp-login.php on sites that are being actively brute force attacked. Just change the permissions on wp-login.php back to 755 and it will execute normally. Change permissions back to 600 when you log out so that your site will be immune to brute force attacks.
So if you see this error in the future, check the permissoion settings for wp-login.php.
Rob