Thank you all very much for all the nice suggestions.
As we are mainly going to use the logo on white paper/print, the black background is not so conveniently.
A white font as e.g. in #33 and #43 is OK though.
Implementing a cross in the logo is a good thing.
At this early stage, the colors are not that important to me. Right now I'm more focused on the overall motive/design.
On the background of the inputs so far, I would say the key words are: A balance between cool, street, playfull and without being too childish.
Rock on! :)