Finally managed to find an error message. after adding debugging info to the _config.php again on the live server - the email one didn't show anything.
define('SS_ENVIRONMENT_TYPE', 'dev');
ini_set('display_errors', 1);
error_reporting(E_ALL);
Director::set_environment_type("dev");
I found the following error code at the bottom of the source:
</html><!DOCTYPE html><html><head><title>GET /Security/login?BackURL=%2Fadmin%2Fpages</title><link rel="stylesheet" type="text/css" href="http://www.example.com/framework/css/debug.css" /></head><body><div class="info warning"><h1>[Warning] Unknown: write failed: No space left on device (28)</h1><h3>GET /Security/login?BackURL=%2Fadmin%2Fpages</h3><p>Line <strong>0</strong> in <strong>Unknown</strong></p></div><h3>Trace</h3><ul></ul></div></body></html><!DOCTYPE html><html><head><title>GET /Security/login?BackURL=%2Fadmin%2Fpages</title><link rel="stylesheet" type="text/css" href="http://www.example.com/framework/css/debug.css" /></head><body><div class="info warning"><h1>[Warning] Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp)</h1><h3>GET /Security/login?BackURL=%2Fadmin%2Fpages</h3><p>Line <strong>0</strong> in <strong>Unknown</strong></p></div><h3>Trace</h3><ul></ul></div></body></html>
[Warning] Unknown: write failed: No space left on device (28)
[Warning] Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp)
Clearing space on the /tmp folder allowed me to login again. This also explains the fact that I could mysteriously login again a couple of weeks ago as the /tmp folder is automatically cleared after rebooting the server.