In two Symfony 2.4.0 projects I get these errors once in a while:

[WARNING 1549] failed to load external entity 
[ERROR 1757] Failed to locate the main schema resource

After manual removing the dev and prod cache dirs and log files I get other errors:

InvalidArgumentException: [WARNING 1549] failed to load external entity 
[WARNING 3084] Element '{http://www.w3.org/2001/XMLSchema}import': Failed to locate a schema
InvalidArgumentException: Unable to parse file "D:\srv\www\sfx\app/config/config_dev.xml".

There are 2 ways I can bypass this error:

1. Restart the server. (Apache/2.4.6 (Win64) | MySQL 5.6.14 | PHP 5.5.6 | Xdebug 2.2.3)
2. In \vendor\symfony\symfony\src\Symfony\Component\Config\Util\XmlUtils.php line 90, comment out "throw new \InvalidArgumentException(implode("\n", $messages), 0, $e);". After this a browser refresh the page loads correctly and I can uncomment that line again.

The full error report is here: http://pastebin.com/jMfdB20u