You'll need to start NodeBB with ./nodebb start not npm
You need to be in the actual folder you installed NodeBB in, you can't just run that command anywhere!
I got this error when logged into the https version of my site while trying to log in at the http version.
I'm quite sure I did, but juste in case I will check again.
thanks, I will reset all plugins for the next update to test which one causes the trouble.
So this sounds like Nginx is up, but NodeBB is down. Check your logs, what do they tell you has happened?