exactly. Besides the base installation and Retropie i only installed git dialog unzip xmlstarlet lightdm openbox and ubuntu-drivers-common. Made that up for a friend of mine who is absolutely clueless when it comes to computers. The idea is "USB stick in, turn on PC = Retropie / USB stick out, turn on PC = Windows"
But i found a solution 20 minutes ago. By accident more or less TBH. I was snooping around in the file manager included in the Retropie menu, actually not knowing what i was looking for and realized that the screen looks different from what i saw on Raspberry Pi. I navigated to dolphin-emu, executed it and voilà, there it is.
Weird... Maybe someone more experienced could satisfy my curiosity and explain why this works through file manager but not through the console.