Vestacp with Nginx - Getting blank pages code 500

Hi, I've setup my Codeigniter app on Vestacp (Ubuntu 14.04) with Nginx front. Most of the app seem to work just fine. But, many pages with or without database query result return blank. Following test record is from apache error log

".. [16/Jul/2015:12:01:37 +0000] "GET /admin/test HTTP/1.0" 500 394 .."

And following test record is from Nginx access log

".. [16/Jul/2015:12:01:37 +0000] GET /admin/test HTTP/1.1 "500" 0 .."

There is not much details available on the access log.


TQ