I made a fresh install of 10.04 which worked like a charm (thanks l3mce!).
I have a problem with outgoing calls. For some reason my area code is prepended to all numbers I dial, which makes outgoing calls impossible. I don't know how the system figured out my are code and why it decided to prepend it.
Since freepbx is no longer available I have no idea how to fix this problem. I would guess I have to fiddle with either the dial patterns or dial rules, but I do not know how to do that. I have digged around in the asterisk mysql database without really finding my way.
Advice would be appreciated! Maybe someone has a link that describes the asterisk real time database? I have searched the wiki without result.