The problem is an unfixable incompatibility between versions of Zend Guard/Zend Optimizer. Zend, for whatever reason (and only they can answer for it) have decided to break backwards compatibility with their newer versions. They are aware of the problem, and have stated that they have no plans to fix it.
x10Hosting dropped their old (SiteReptile) site builder because of this issue. If you built your site using SiteReptile, there's not a lot you can do other than to back up your site locally and install it on a machine that uses PHP 5.2 and the old Zend Optimizer, then recover the pages using VIew->Source or something similar.
If you are using a commercial or freeware script that used Zend Guard to obfuscate the source, then you'll have to try to find an updated version of the script that will work with PHP 5.3.