CB field with CB Conditional shown on and off in edit mode

Are there any conditions on your radio field that's storing Y and N? 
=> yes, see the last screenshot I attached days ago on your request. It is since I added the second condition, that the change is not saved to DB anymore

Is there a reason you're using a radio field here instead of a checkbox field (this can be set to Yes/No buttons mode)?
=> no other then an esthetical reason. I created now a checkbox field as a copy: this also does not store any change from 'Yes' (1) to 'No'(0) in the same condition,
formulated in words: previously when there were less then 40 subscribers who selected it, I set it to yes, and now that the maximum of 40 is reached and it is not shown anymore to new people, I who selected it before and can still access it, can not unselect it anymore, my change is not saved.

Ok, so it looks like you have the field conditioning itself then. You can't do this. That will condition away the field before it can even process. You can try doing it with a static condition like the following, but a field should never condition itself as that creates a loop.

Field: Custom > Value
Custom Value: [FIELD_NAME]
Operator: Not Equal To
Value: Y

That's a static condition and they process differently, but it likely won't matter as again a field shouldn't be conditioning itself.

Ok, that is allready my second condition. So it means it is not possible.
I tried to set up a subscription with for an event that can have maximum 40 participants, once 40 reached, the item should no longer be offered. But since I will always need to refer to that field itself, the conclusion is that this is not possible in CB.
Thank you for your time
 

Your second condition has the field selected from the dropdown. That'll create a dynamic condition. Try the static condition provided above which utilizes a substitution. Aside from that I don't know what more to suggest as a field typically cannot condition itself during profile edit or registration, but it's fine to do so for profile view.

Now I am happy
It works fine with the static condition, exactly as I need.
Thank you so much for your help!