500 Internal Server Error after logging in
From my research there are basically four things that could cause the '500 internal server error' with drupal : 


Misconfgured server (you will have to dig into your server configs)
Misconfigured .htaccess file (replace your existing .htaccess with the drupal default)
Misconfigured file permissions
Low php memory_limit setting


In my case, the culprit was the memory_limit setting. After doubling the same, my site is flying right now.