The area was actually not being filled with plastic. But that was the clue. For some reason the interior infill was set to 0. That's why it failed. Stupid me.
Thanks for the assistance!
I've got this lamp head I like to print. It needs some supports. Most of them are generated as expexted, but some of them are not. I cannot figure out what I am doing wrong here. The angle is nearly 90 degrees, so I would expect them to generate. The structure needs supports between the layers 0.5mm...