@julian, I think I fixed this issue. I rolled back to a snapshot before I installed NodeBB and re-cloned the 0.5.1 branch and it installed without an issue and I have sweet, glorious CSS!
When reloading a website while writing a post, the title of the post doesn't get saved
OS: Ubuntu 16.04.5 LTS
Language: German (de-DE)
Installation directory: /srv/nodebb/
Guide used: https://docs.nodebb.org/installing/os/ubuntu/
When reloading the website (NodeBB) as a client while writing a post, the title of the post doesn't get saved, but the content does.
You should really be submitting bug reports to the NodeBB GitHub page, or the respective plugin page