luceos Well, actually I have gotten that problem when I used Flarum with Heroku.
Anytime I post a new discussion, gives me the error. If I refresh, the discussion will exist, but it will be deleted, and will contain no posts whatsoever.
The problem in my case was that Flarum was trying to open a file, but because of permissions, couldn't and gave the error null.
Where are you hosting Flarum, @videosurfer?
PS: you can test to make a new discussion with mine @ http://discussion-flarumjs.herokuapp.com with username usernames and password password