Sorry I always forget to post the version / hardware 

On that m0n0wall box 

I use 1.3 with Compaq  733 MHz 128mb RAM onboard Intel 82801BA/CAM (ICH2/3)
Pro/100 Ethernet  as WAN and 

3Com 3c905C-TX Fast Etherlink XL as LAN interface.

I have about 18 m0n0wall boxes running same version and 1.3b13 I did not see
that error before.