I ran into the below error today while working with Magento 1 and registering a customer during checkout.

[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Uncaught exception 'Exception' with message 'Warning: mcrypt_generic_deinit(): Could not terminate encryption specifier  in /var/www/myproject/htdocs/lib/Varien/Crypt/Mcrypt.php on line 135' in /var/www/myproject/htdocs/app/code/core/Mage/Core/functions.php:245"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "Stack trace:"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "#0 [internal function]: mageCoreErrorHandler(2, 'mcrypt_generic_...', '/var/www/myproject...', 135, Array)"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "#1 /var/www/myproject/htdocs/lib/Varien/Crypt/Mcrypt.php(135): mcrypt_generic_deinit(Resource id #180)"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "#2 /var/www/myproject/htdocs/lib/Varien/Crypt/Mcrypt.php(54): Varien_Crypt_Mcrypt->_reset()"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "#3 [internal function]: Varien_Crypt_Mcrypt->destruct()"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "#4 {main}"
[10-Apr-2018 13:49:47] WARNING: [pool www] child 1731 said into stderr: "  thrown in /var/www/myproject/htdocs/app/code/core/Mage/Core/functions.php on line 245"

The problem is the encryption key in app/etc/local.xml was empty. Add one to solve the problem.