Nice! Thank you @Junkhacker and @george1421
I just changed it to work with the clients MAC address and it seems to work like a charm!
So now, i have this config:
iseq ${net0/mac} XX:XX:XX:XX:XX:XX && goto chainboot ||
goto fog.local
:chainboot
set pxeserver:ipv4 XXX.XX.XXX.XXX
set next-server ${pxeserver}
chain tftp://${pxeserver}/pxelinux.0
exit
I will do some testing but i am verry optimistic 
Thanks!
I did the same with the same error … Fresh installation of ubuntu with a fresh fog server. I will try it again on other hardware and another setup later this week.