Nginx and broken images

I am testing a new server with nginx and SSL, inspired by https://github.com/perusio/piwik-nginx

So far it works, but some images do not load in the admin pages, where the image URL is relative like:

src="plugins/Morpheus/images/goal.png"

while the ful image URL loads correctly, such as:

https://example.org/plugins/Morpheus/images/goal.png

Do you know what causes this issue?

I ended up ditching the perusio config and using a standard ngxinx php-fpm configuration, everything runs fine now.