I have found the error in the amp circuit, and I am rather embarrassed about it... Take a look at the pictures below to see if you can spot the mistake faster than I did. the components and input signals are fitted closest to the camera:
So, what is it? I soldered the components to the wrong pins! When inserting a jack, the connectors are lifted, and there is no more connection between the jack and the signal pins. Why I could get any sound at all is beyond me!
mark: I will not use this crappy thrown together output circuit in my final design, but it's nice to have while waiting for the real thing.