I was baffled by my disksystem all the same, telling me a cp -r /home would not work cause there is no space left (moving 840MB to a "2.9GB free" partitition).
The meat of the story is: du did not find a subdir "Öffentlich" and did not count the files inside it.
For those interested - thats what I found :
du told me 840 MB used (Disk Usage analyzer the same)
df told me 5,3 GB used
I found various tips revolving around
lsof -n | grab deleted (to find out wether there are any "deleted yet open" files hugging space)
sudo tune2fs -l /dev/sda1 | grep 'Block size' (to find out the block size 4k in my case)
find /home -size -4k | wc -l (to find out the number of files smaller than 1 block, a 10 bytes file using a 4046 block is wasting lots a space)
du -sh ./* (finding the space used (real size) in each subdir)
df -i (finding not the number of used blocks, but the no of used inodes, whats it worth for, I am not sure yet.)
Further tips found :
Disk Usage Analyzer is rights-aware - "sudo baobab" will show more than "baobab"
du -c --max-depth=4 /dir | sort -n
ncdu utility found at: http://dev.yorhel.nl/ncdu
The best tip for me was du -sh. It helped me find following directory, which was not "cumulated" into the du / sum.
/home/mine/pictures/Öffentlich (german O-Umlaut)
At first I thought that was for the "O-Umlaut", but copying /home to a new partition solved that for me.
Good luck hunting.