PHP "Exception not found"

I have a somehow funny issue. While trying to understand why a certain website returns http code 500 to browser, I found the message

PHP Fatal error:  Class 'MZ\\MailChimpBundle\\Services\\Exception' not found in /var/www/website/vendor/bundles/MZ/MailChimpBundle/Services/MailChimp.php on line 41

in apache log. Looking at the mentioned line:

throw new Exception('This bundle needs the cURL PHP extension.');

I now understand how to get the site working, but I still wonder why the code for throwing the exception (which would have resulted in a more helpful log message) failed. What could be the reason?