Problem solved. It was the nodebb-plugin-cash who caused the problem.
After deleting this plugin my NodeBB worked again 🙂
Hi all, this is indeed a dumb question but better asking and sound dumb rather than taking dumb decisions.
I'm thinking to buy the following option and I wonder if it would be enough to run a forum with a peak of 600 concurrent users.
I'm pretty confident that the number of concurrent connections to Mongo DB (max 500) has nothing to do with the number of users connections to the forum, but would be great if somebody confirms based on his/her experience.