Finally, I found a workaround. Dont't know if it is the way to go - but at least it works for me:
I created a shell script enableMultitouch.sh as root in the directory /usr/local/bin/ and made it executable. (As described here: http://wiki.ubuntuusers.de/Autostart#Autostart-ueber-Skript.)
I pasted the commands as mentioned in the post that described how to enable multi-touch:
xinput --set-prop --type=int --format=32 "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Pressure" 4
xinput --set-prop --type=int --format=32 "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Width" 8
xinput --set-prop --type=int --format=8 "SynPS/2 Synaptics TouchPad" "Synaptics Two-Finger Scrolling" 1 0
As described here: How to enable Two finger scrolling on Lenovo Thinkpad T400 in Ubuntu 12.04 64 bit
Finally I added the script to the startup applications. And now two-finger-scrolling works just fine.
Hope this is somehow helpful...