Gordus,
is it leaking at the cap threads or the Festo connection?

I have a brand new Hansen Trap I just pulled out of the bag, the cap threads have Teflon tape applied, mine does not have that dark object like is shown in your photo, did you take the cap off to investigate?

The only O-ring that should be in the cap is the seal for the Festo connector.

Here are some photos of mine....good luck