grml-chroot does not properly handle the situation when the user decides to 
chroot into the same chroot multiple times (for example, in different windows). 
When the first chroot terminates, the unconditional umount leaves the other 
chroots without mounted /proc, /dev etc.

I would suggest touching a file inside the chroot prior to the chroot, removing 
that file after the chroot command has terminated, and only umounting the 
filesystems if no other file is in there.

Greetings
Marc