m0n0wall Forum - DHCP relayer eats DHCP OFFER ?

We have a problem with DHCP relaying.

The DHCP DISCOVER broadcast is relayed correctly (as a unicast packet) to the DHCP server, and the DHCP OFFER reply is sent to the monowall (to the networks gateway address), but it doesn't arrive on the client Sad

There is nothing interesting in the system log or the firewall log, and we've tripple-checked all rules (they're pretty simple).

hardware: Soekris net5501, software: monowall 1.33 (latest version).

The relayer is enabled on two vlans (LAN and PUB) on em0 and on a net on
vr3 (COM), and the DHCP server is on a network routed via vr2 (VPN Tunnel).

We have another soekris, running monowall 1.31, where the DHCP relayer works.
There, the DHCP server is behind the WAN interface, though.
The relayed DHCP traffic from the working monowall is routed through the non-working monowall (to the same DHCP server).

Any ideas ?