Thanks for the reply!
Today I tried again to connect with ".../json/all" and it works.
The only difference compared to my first post is that I restarted the UniPi couple of times whereas at the time of my first post the UniPi was online since weeks.
We are using UniPi Neuron L503 (S/N: 215) with Neuron xS40
Evok PrecedingRelease:v.2.0.7h PrecedingRevision:379
I installed Evok manually
Thanks and greetings