Yesterday, I ran apt-get update && apt-get dist-upgrade (have done that before this) and the touchscreen's (or rather the mouse pointer's) Y-axis got inverted after a reboot.
I tried adding Option "InvertY" "1" to /etc/X11/xorg.conf but no luck after a logout or even a reboot.
(I did then try "InvertX" "1" and even "SwapAxes" "1" just to see if there'd be any change but got nothing.)

After clumsily navigating to Settings I managed to start also the Calibrate Touchscreen but that didn't help either.