Trigger Warning: I found a solution but I feel this is not the right way to solve it. 
I did what is described here: Upgrading Your Project to UE5 | Talks and demos under the headline " Changing Your DefaultEngine.ini". So I first changed the global illumination method on the config folder manually. And this actually changed it to lumen also in the editor. But I still was not able to click the drop-down nor change any other lumen settings. Then I wanted to change the other settings as well this way. But smart as I am I just copied the settings seen on the site above. Afterward I read that these settings should be added to the ones already there… :D. But turns out now it works. So I can now change the settings in the editor. Don’t really know what’s going on. But I just added the settings I deleted back on in the editor. And now everything seems to be working fine. I am still interested in how this should be solved appropiate tho
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