Hi Markus,
I had this same issue a few evenings ago when setting up my box, after I'd done all my settings. It didn't work.
I decided the next day to give up and reflash the box with 2.3 and installing CCCam was the first thing I did and it worked. I then just redid my settings. I had everything going again within 40 minutes, also working with my official card.
It may be a bit of a pain to reflash, but it certainly worked for me. Note that when you start installing the local extension, the window should actually say "Installing xxxxxxxxxxx.ipk" or something similar. Mine wasn't when it was failing, but did when it worked, so I guess that's your clue before you reboot.
Hope this helps,
Silent.
Edit: I should have read your entire post to see it worked with an older version! I wanted to install the latest CCCam, which is why I didn't try an older version and opted to try reflashing the box.