@LineOf7s Well then, probably the fact that my USB stick is formatted as exFat.
I guess I'll give it a break with Amiberry for a while.
Anyway, I am just updating my system, including the big one (second list item from the RetroPi-Setup Script). Given I get my PS3 controllers working on Bluetooth again afterwards, maybe Amiberry also behaves differently :-)

@mitu

i've tried that already with no luck
the resolution within the black bar will change, but it wont show the full screen

the smaller Res i go, the more the image gets cut out within the black bars (kind of like enlarging an image, or zooming in with your phone by using two fingers)
and the highest Res it will go (1920x1080 at 60hz) will not display the entire image