Ok folks,

I fixed it in that way that I connected my m0n0wall-PC with a KVM - it
worked!

No idea why it doesn´t worked with a normal PS/2 keyboard connected.

When m0n0wall booted the following messages came up:

...
atkbdc0: <Keyboard controller (i8042)> at port 0x60,0x64 on isa0
atkbd0: <AT Keyboard> flags 0x1 irq 1 on atkbdc0
device_probe_and_attach: atkbd0 attach returned 6
...

I checked some other mailinglists where people had trouble with FreeBSD and
their keyboards,
but no solutions so far for me.

I tried a file "device.hints" in /boot but I guess FreeBSD 4.x doesn´t know
of such a file.

Thanks for your suggestions.