Google App Engine PHP无法解释500服务器错误

I'm running a Zend Project on GAE (PHP 5.4). On my development environment, everything works fine. When I deploy the app, however, I get 500 errors on form posts. If I refresh the page, the data gets posted, even though the form validation fails. In both cases, the request only takes a couple of seconds to return (leading me to believe that this is not a timeout issue).

I've had this happen consistently, but the 500 has only been logged once:

Logging Severity: Warning
A problem was encountered with the process that handled this request, 
causing it to exit. This is likely to cause a new process to be used
for the next request to your application. (Error code 204)

Here's the response header from one of the times I got the error:

HTTP/1.1 500 Internal Server Error
status: 500 Internal Server Error
version: HTTP/1.1
alternate-protocol: 443:quic
content-length: 466
content-type: text/html; charset=UTF-8
date: Tue, 12 Nov 2013 06:20:38 GMT
server: Google Frontend

Any ideas?? This is driving me crazy!