So i managed to get both problems fixed:
1. I deleted all shares and readded the shared folders, now i don't get the GUI error anymore.
2. And with these tips i got the upgrade to linux 188.8.131.52:
Remove the line which says APT::Default-Release from /etc/apt/apt.conf and run sudo apt-get update. It should now see the backports kernel. I suggest you install sudo apt-get install linux-image-`uname -r`.
Unfortunately the wifi card doesn't work yet, but i guess i will open another thread in the correct subforum.
Thanks for your help!