Magento Community Edition 18.104.22.168, CentOS 5.7 64-bit
In the Magento admin interface, you click on System/Configuration and then choose the Shipping Methods option from the left column.
If you try to change any of the shipping methods, such as disabling Flat Rate shipping or enabling Table Rates, you get a nasty 500 error on save.
This error is caused because mcrypt is probably not installed on your server. Mcrypt is a PHP extensions and if you bothered to read it, the Magento documentation does say that it is a required part of your server configuration. Don’t worry, I didn’t either 😉
Let’s install mcrypt and get on with our awesome ecommerce site!
SSH into the server and run the following commands:
rpm -Uvh http://dl.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
yum install php53-mcrypt
The first line tells yum where to look for mcrypt, otherwise it will tell you it was not found, and the second line does the magic.
Once mcrypt is installed, restart the server and the problem should be resolved.