Same issue here, but problem is only present when external interface and soft AP are both present on open networks.
Once wlan0 is associated and has external acces...
iwconfig wlan0 channel 8 mode managed ap auto essid SuperSecretNetwork
Comes up without issue, so...
airmon-ng start wlan1
Everything good so far, so...
airbase-ng -P -C 30 -e "Free Public WiFi"
At this point everything should be good... but it isn't. Packets coming into at0 are 36 bytes short, and misidentifying as FC packets instead of Ethernet. If the entire process is done using wlan0 on an encrypted connection, however, the whole setup works perfectly.
ifconfig at0 IP.ADD.SS.1 netmask 255.255.255.0
ifconfig at0 up
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o wlan1 -s IP.ADD.SS.0/24 -j MASQUERADE
service dhcp3-server start
Anyone have a thought?