So I just cloned a drupal site onto a new server. It all worked fine but then I couldn’t log in. I found this post which said you need to perhaps change the $cookie_domain variable but that didn’t make any difference. Finally I found the root cause of the problem – mod_rewrite wasn’t active so even though the user/login page was displaying (it was status 404 which redirects through to index.php hence into drupal) it wasn’t accepting POST requests.

a2enmod rewrite
service apache2 restart