Sorry that I had to repost this again. I took a closer look at what happened
and I noticed that there is a neutral wire also going out from the switcher
PSU. this is what touched the metal body of my amp. So basically the amp was
connected to the PSU and also the ground was connected to the Neutral of the
mains.

I have not yet checked the diode but this is where I will start. The burnt
resistor is a 1/4W one so its not a power resistor. There is also a
transistor close to the burnt resistor that seems to have taken some heat.
All smoothing caps seem fine.

I just received news that there is no warranty on the amp so I can now start
working on it myself.
Any other Ideas?