Using unbundled nginx breaks gitlab installation

I recently set up a gitlab ce installation for personal use and test around a bit.
My server (Ubuntu 18.04) already provides a running nginx server with a few pages on it. Since that conflicted with the standard omnibus gitlab installation, i moved gitlab to port :87 just for testing purposes.
Now I’m trying to move gitlab into my existing nginx configuration, where I’m getting a passenger related error.
I followed the official guides:
For my passenger/ruby installation I used this guide:
As far as I can tell, I can reach gitlab under my specified URL. But the site will only load up to the point, where nginx/passender should spawn a process for gitlab itself.
Could not spawn process for application /opt/gitlab/embedded/service/gitlab-rails: A timeout occurred while starting a preloader process.

I already removed and reinstalled gitlab quite a few times, but I always end up at that point.
Before I try to use my existing installation it runs without problems.

I hope this forum is the right place to ask, since I assume my problem is related to nginx and/or passenger and not gitlab itself?

Kind regards