Ok, I've made some tests. I've always booted using the CD.
USB Stick recognized USB-Mouse and USB-Keyboard System boots
by BIOS plugged in
-----------------------------------------------------------------------------------------------
yes no no
no yes no
no no yes
In the first two cases the system didn't boot, when there was a USB device already attached to the computer, when the computer booted. Those setups didn't work.
When there was no USB device attached to the computer and I booted into plop, then plugged in my USB-stick, and tried to boot USB it worked.
I'm not sure if this could be some BIOS problem...
PS: Do you speak german?