Custom fields resolved - it's a typo in the example file!
it had addpath= near the top instead of addfieldpath=
Now Custom fields are shown again!
Unfortunately you still cannot choose a value
I always get the "mandatory field" error for all of my mandatory custom fields when I try to save the event :/
And another question: The Documentation says, you can define what value should appear in the "Custom Layout" for Event List or Event Detail if the value is not set. How do I do that (for example for a checkbox).
What I need is:
Checkbox not checked: - [No value - empty space]
Checkbox checked: value XY