after update the store from 1.8.1 to 184.108.40.206, when i add to cart a configurable product and i logged in, the system return the page 503.
The same operation, but if not logged in work correctly. The system.log and exception.log don't report an error related to this issue.
Our hosting provider, believe this error is caused by a bug in PHP/Zend framework. We have tried with PHP 5.5 as well with same results so at this point there is not much more we can do.
A bug report has been submitted: https://bugs.php.net/bug.php?id=68958
all suggestions are much appreciated.