Hi! Updating to 2.21 (update via source) solved the problem -- thanks! I could even delete the "custom" configurations for the games I tried before, Amiberry now sets port 0 to mouse and port 1 to joystick all the time -- thank you!
Now, everything in Amiberry works fine for me, there's no lag in games, the graphics look good, the loading times are fine... Just one thing is broken -- the sound, see https://retropie.org.uk/forum/topic/19528/broken-amiberry-sound . Maybe someone can help me out there, too?=
Thanks!