@PitaJ NodeBB is installed on a server and I'm using a proxy to serve it.
Both servers are on Heroku on free Dynos. If nodeBB is sleeping, the proxy server will wake it up, but when it starts it throws a socket error and the proxy server displays the "Application error" page.
But in reality NodeBB server started okay.

@PitaJ and who else may encounter this issue, here is how I solved it.

The root problem was the image format used for "Site Logo" in general/settings.
I've uploaded a png file and the information was not saved in the database, althoughthe fields remained filled across the sessions.
Once I've changed the site logo with a jpg file, the emails were properly formatted too.