Solved! Well, Uberspace ...
Hope that will help other users to run NodeBB with Uberspace. The problem was the run-file I used for the daemon.
This is my new code. ([USER]/etc/[YOUR-SERVICE]/run😞cd $HOME/[NODEBB-FOLDER] ./nodebb start
In the official NodeBB documentation the preffered method to start is ./nodebb start.
Don't follow third party tutorials in this case ... NodeBB seems unstable with other methods to start.
I will test it further, but for now it seems to work pretty well.