Thank you, this did do the trick!
However, now the pipeline does not go to the PLAYING state anymore (unless I remove the branch with the pngenc entirely from the pipeline). Do you, by any chance, have an idea what might be the cause in this case?
I saw in a different post (http://gstreamer-devel.966125.n4.nabble.com/tee-won-t-go-in-playing-state-td4680128.html) that it seemed to have something to do with the x264enc requiring 3 seconds of material and possibly getting stuck there because of the queue on the other branch not being able to hold that. However, I have added a tee_queue.set_property_from_str("Queue-leaky", &"downstream"); to that queue to ensure it will just drop frames if it’s full. I can’t tell if that does work, however, as the set_property_from_str function does not provide a result.