The spar box channel was leaning on the hinge which needed some more investigation.
On blogs of other builders some clearance was visible.
I thought about removing 2 mm with a 1/4 inch drill, like this:
After consulting TAF I learned that on factory built Slings the spar box channel is also leaning on the hinge, without clearance. But TAF approved creating some clearance. This is the result (see also the thread seal on the 5 bolts):