502 bad gateway

Wed Dec 9 16:03:56 UTC 2015

Hello,

I have a magento website working fine for months now.
However i recently installed a PDF plugin to send invoices with emails.
The PDF url is giving a 502 bad gateway
"{admin}/sales_invoice/view/invoice_id/717/key/5650a40d93929f5db4383db2a169b08a/".
I have checked around on the internet and cant find the issue here. no
APC. increased fastcgi buffers doesnt change anything. checked nginx
logs, magento logs and user logs but nothing.

Could anyone point me in the right direction on how to figure this one out?