Linux

Then you need to take a closer look at the USB stick's boot process. Just kexec-ing a kernel and mounting sdb1 might not be the way to go. It may be mounting some image inside the DOS filesystem or something. Or it may be running purely from some image in RAM for all I know. I really can't tell what it's doing from here but it's sounding weirder by the second.

I'm not convinced your device files in the initrd are even right. It's quite possible for the device file to exist but the device not to exist, which would cause 'invalid argument' or 'no such device'. Look under /sys/class/block/ and post what's there.

Good to know!
Would changing the filesystem change anything? It doesn't matter which I'm using I just had my Stick formatted in FAT32 and leaved it like that...
As said before I will try your suggestions friday, I don't have access to the system until then, sorry!

Depends entirely on what it's doing. But it doesn't sound like it's booting the normal linux way, there'd be trickery necessary to get a bootable Linux system out of a FAT32 partition.

What files are inside /dev/sdb1 ? I suspect there's a clutter of small inscrutiable files (bootloader stuff) and one great big file(disk image).

When the initrd crashes again, try cat /proc/filesystems to see what partition types your kernel really supports. Also try just running blkid to see what disks it thinks are really disks.

blkid says my stick is /dev/sdb1 and his fs is vfat
/proc/filesystems doesn't have vfat...
Reason enough to try it again with ext3^^