Whats odd is that Kirby is quite happy to create the media folder, and generate thumbs in it, but not store the panel assets in it - so it must have permission.
The panel assets cannot be installed properly. Please check permissions of your media folder.
What gets even weirder, is that if I push the site up with rSync, with the panel assets already installed, it still gives me that error. I guess it does not check they are already present before trying to create them, but thats probably on purpose since I guess they may change with each version release and you dont want the possibility with an older set being used with a newer panel.
Feel free to split this thread, since this is a different issue now, and the answer might help someone else.