So something broke between version 2.6 and 2.7 of wpa_supplicant.
Downgrading did the trick.
I'm running arch (surprise something broke), so I did the following:
sudo pacman -U /var/cache/pacman/wpa_supplicant-2\:2.6-2-x86_64.pkg.tar.gz
echo 'IgnorePkg=wpa_supplicant >> /etc/pacman.conf'
After fiddling with this for a little longer I found, that ...
create_ap use 192.168.12.1 as default gateway. check it:
The -g option can be used to set a custom getaway:
sudo create_ap -n WifiInterface factory-tester -g GATWAY_HERE
IPv4 Gateway for the Access Point (default: 192.168.12.1)"
To get the IPs of the connected devices: