I reflashed now 4.4.0.64, @piggz 's port works like a charm, but every attempt to update ruins the OS, at least makes it slow, at worst leads to total crash. Now I leave it as it is and have installed a minimum of apps and will look for a mini-laptop for portable work and use the phone only as WLAN access point. I’m sick of permanently struggling against unnecessary limits e.g. the tiny root partition or have to fear that an update ruins everything and I have again to work a whole day to restore it.