For those who want to try the same:

The first active bootloader (which generates the menu with the F-keys) comes from FreeBSD in the first primary partition mbr.

The other one, grub, comes from knoppix-hdd-install and is located in the 3rd primary partition.

There seemes to be no way to boot directly from the FreeBSD bootloader cause that one can't boot linux.

It took me some time to recognize what is happening there... Maybe it helps you when you want to install a *BSD next to windows and Linux.

Greetings
Daniel