What do I do with a “fatal error – internal zend error” ?

“Zend engine” is the engine in the guts of PHP (i.e. the language that WordPress is written in).

An “internal zend error” means a PHP bug. i.e. Not a programming or application issue, but one in the core of PHP itself.

To resolve this, the usual solution is to upgrade the version of PHP that you are using. Normally,such errors are soon found, reported to the PHP project, and fixed in the next release. So, if you encounter such an error, the chances are very good that it’s already fixed in a subsequent PHP release. If you do not know how to move to an updated PHP version, then you will need to speak to your hosting company or server admin (the answer will vary between setups, and sometimes they might need to perform the upgrade themselves).