I do have found my question before but none of the other solutions seems to work for me.
I have installed Netdata a few days ago and it just worked fine. After installing Nginx the web interface stopped working.
I am using Google Chrome Versie 90.0.4430.212.
For now I only want Netdata to run on my Dietpi.
I have to say I am pretty new to Linux so if someone could help me out that would be great 
(so if you are willing to help me keep in mind you are talking to a noob 
)
Just ask if you need me to provide more information about the problem.
Let’s see, please do the following commands and share the result
# ssh to the Dietpi server
ps faxu | grep "[n]etdata"
sudo ss -ntlp | grep 19999
It started working randomly after I restarted my PC several times. But thanks for your answer