[Grml] vim, grml 0.9 on hd - strange characters after using backspace

I have a strange problem with vim. When I use backspace to delete some characters while I am in instering mode, there appear blanks or other characters which I definitely not typed.

example:

[a], [backspace], [a] results in the folowing character

å

Maybe it has something to do with unicode. I use unicode_start in /etc/zsh/zlogin.

Regards, Frank