500 Internal Server Error after logging in I have a D7 site. I start getting a 500 internal server error. I get the default Chrome (my browser) error page, instead of the one from my server. 

This error occurs only after I log in and have navigated through a page or two. The apache error log shows nothing except for the following message : 

[Sun Jul 15 21:53:27 2012] [debug] mod_deflate.c(615): [client 127.0.0.1] Zlib: Compressed 0 to 8 : URL /php5-fcgi/index.php, referer: http://new.whack.co.in/admin/modules


I can browse through the whole site anonymously without encountering any problems. I can even access the site temporarily by clearing my browser's cache.

The site uses a combination of Varnish+Apache 2+APC+php-fpm+memcache.
I have a D6 site installed on the same server and I can navigate through it without encountering any problems.

This site was installed by using Drush Site Upgrade. Contents and settings were migrated from a the previously mentioned D6 site. The D6 site is stored on /var/www/drupal (Accesible from default domain) while the D7 site on /var/www/seven - (accessible from a subdomain).

What can be the possible cause of the problem and how can I fix it? Please do let me know if any other information is required.
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.