@oplik0 thank you for your answer. Traffic management is already on, because we saw a lot more 503 before this was on. Also we reduced the session time down to 3 days.
A bit backround infos of the forum:
I dont know if 400 up to 1500 connections permanent is massive for nodebb or not.
We have around 1.2 million site calls per day.
Our user base growing permanently since we have started our project back in 2015, actually 21400 users are registered. Per week 100+ new registrations actually, during Winter this rate will increase.
The most of the users are from countrys with german language, but also english, dutch, russian and portuges are on our forum.