Zyxel NBG6817 problems flashing OpenWrt

Hi,my post might not quite fit in here, but since I didn't find an active thread for the Zyxel armor Z2, I'll try it here.
My internet access is about to get upgraded from 32Mbit/s to 1000Mbit/s so I thought I'll finally switch from my netgear wndrmacv2(openwrt) to the zyxel nbg6817.
I already flashed the zyxel with openwrt in the past and tried to update it with some self compiled snapshot.
Unfortunately it's not working - tried it with luci and with sysupgrade, but both attempts failed-the zyxel reboots an shows the old image still active.
Right now it's running with OpenWrt SNAPSHOT r10766-3032bf7f89 / LuCI Master (git-19.228.15332-c2d6770) Kernel Version 4.14.138.
Am I missing something here?
Should I use tftp to flash with the latest snapshot?