After fiddling around with /etc/fstab and the like my boss suggested to try fsck.
sudo fsck /dev/sda5
"Fix file Y?", yes. "Fix file Z", yes. This went on for a few dozen files and finally I rebooted my laptop and everything was fine again.
However, beware of this operation! In my case a few installers, like Plants vs. Zombies, went missing ( :( !). If you care about all your files, do a backup first.
Oh, I hear some of you asking "How the hell is that possible when the /home partition isn't mounted?"...
sudo mkdir /media/home; sudo mount /dev/sdaX /media/home
And how to find out the partition's number (sdaX)? Either use gparted or sudo fdisk -l