Installation not allowed by ".." slot rule of interface "serial-port"

hmm, indeed … most likely the device add even happens in the initrd on boot so it is already plugged … you could add an additional oneshot daemon.

that would run exactly once on boot and force the initalization…

I have added the udev rule in /etc/udev/rules.d/icse012a.rules and now it seems to work.
I have removed the additional daemon.

randomly adding udev rules wont be an option in Ubuntu Core, so this is only possible if you can actually ssh into the device and scp the rule in place (or create it manually) … if you only do tinkering for development this is indeed fine, for a final product based on UC it wont work though …