NPF disables itself/instability

[Benjamin Kiessling <mittagessen%l.unchti.me@localhost>]

Hi,

I've recently installed a NetBSD (6.1) on my home router
(alix2d13/i386/VIA ethernet NICs) and I am encountering some weirdness
with NPF. After booting it disables itself automatically without
discernible reason, i.e. I run

# npfctl reload
# npfctl start

and after 1-10 seconds the output of npfctl status shows that filtering
is disabled. After restarting it a few times it stays activated. Kernel
log and syslog doesn't show anything.  This is my first serious NetBSD
box and I'm completely at a loss here. 

Regards,
Ben

PS: My configuration

map vlan2136 dynamic any -> 146.0.105.102 pass from { 172.22.94.64/27 } 

group (name "external", interface vlan2136) {
        pass stateful out final all 
        pass stateful in final all 
}

group (default) {
        pass final on lo0 all 
        pass all 
}