The very nature of the 500 null error makes it hard to debug. This occurs (from what I can figure) when an error gets thrown so early in your application that nothing ever gets flushed to the browser. The "null" part of the error probably means that no data was received from the browser.

In order to debug this error put a CFFlush tag right after your application declaration (Application.cfm) or somewhere very early in the code. Once you get the data stream going, you sort of let gravity take over (metaphorically speaking). Now, the 500 null error will be replaced with the CFError that is actually being thrown.

CAUTION: Once done, REMOVE THE CFFLUSH. The CFFlush tag will kill of your CFLocation tags as they work via page headers (which are not settable after data has been sent to the browser).

refer this link

http://www.ehow.com/info_12103190_causes-jrun-servlet-error-500-null.html

Could you show us the content of your Application.cfc file?