nginx + php 5.6 serving empty pages, can't find error log

I have accidentally hosed a VPS which was hosting a website using nginx and PHP 5.6, and I’m attempting to recreate things on a new server running Ubuntu 18.04 (previous one was 12.04 – upgrade failed badly). Because of compatibility issues I have to use an older 5.x PHP version. I have got things working so that a simple test page is working. But in any of the pages using the Couch PHP CMS package, I’m seeing a completely empty page and a 500 error recorded in the nginx access.log, but nothing in the nginx error.log, nor in php5.6-fpm.log or syslog. So where might errors be written to? I’m hoping that if I can actually see what is causing the 500 error it should be easy enough to fix.