500.shtml error? What is this? Help!?

I've deployed my project to a client's server and am getting a strange
error when submitting text via a simple form. This does not happen on
my staging server. The full error looks like this:

NOT FOUND

Error: The requested address '/500.shtml' was not found on this
server.

Obviously I'm not looking for a file 500.shtml...just submitting to a
controller action. I've read somewhere about Zend Optimizer causing
problems with CakePHP. But Zend is disabled on this sever? Any ideas?

Thanks.
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