Thread:Project:Support desk/SpecialPages gives 500 Internal Service Error - Premature end of script headers: php5/reply

https://www.mediawiki.org/wiki/Manual:How_to_debug#PHP_errors explains how to get more verbose PHP debug output. And FastCGI is mentioned in http://stackoverflow.com/questions/4336757/500-server-error-premature-end-of-script-headers