UPDATE: It works now
I don't know why but I'm pretty sure it has something to do with the connected Thunderbolt Display. As soon as I've disconnected the Thunderbolt Display it worked flawlessly! So I need to be disconnected from the Thunderbolt Display!
Just to be clear: I always put the CC3D directly into the Macbook's USB 3 port, never on the Thunderbolt Display's USB port. Though, the display had some kind of interference with GCS.
I think there aren't a lot of LibrePilot users with a Macbook and a Thunderbolt Display out there, but this post might help them
Of course I would like to know where the problem was and I would love to help you guys debug it. I'm a Python developer and Linux systems engineer myself, so let me know if I can do anything to pinpoint the issue.
Cheers
Domi