I have solved the problem (I think). I wouldn't call it a bug in m0n0wall,
but rather a side effect of how stateful firewalls work.
By executing the command:
echo '@67 pass in quick on dc0 from 192.168.1.0/24 to any' | ipf -f -
it suddenly works. The other rule that should have allowed the traffic:
@2 pass in quick from 192.168.1.0/24 to any keep state group 100
Doesn't work because of the "keep state" (or "group") and as I described
before is has no state for the packet that arrives later so it is dropped.
If someone can explain the exact cause of this I am very interested.