Thank you, I did that - using a shell like /bin/bash works fine.
However, using the "chroot" command without "su - username", my user writes all his files as root, not as "username". Thats where I need the "su" for, which is not working...
EDIT: Sorry, some more context: I need sudo rights to use the "chroot" command, afterwards I need the user to switch back by using "su - username". This is where my problem is.