Oh it looks like a problem with one of my plugins https://github.com/barisusakli/nodebb-plugin-twitter-hashtag.
Confirmed, that was it, thanks. I hadn't realized that that had gotten installed.
Title says it all. For some reason NodeBB doesn't works with synchronous DFP code with synchronous rendering.
I already contacted Google DFP support and they couldn't really help with it. I think that this bug should be definitely noticed as montization is a major point for webmasters.
I am running the adsence plugin only because there are so much trouble with DFP and NodeBB