aptitude -r install linux-headers-$(uname -r|sed 's,[^-]*-[^-]*-,,') nvidia-legacy-340xx-driver
I may have done: aptitude -r install linux-headers-$(uname -r|sed 's,[^-]*-[^-]*-,,') nvidia-driver instead of the mentioned first. Did it again, now it worked. This bug can be closed - thank you for your support ! herrdeh