I do not think this is Openhab related.
Is your USB-to-serial properly configured (dmesg + lsusb) in Linux and is it a good cable (some converters do not work properly)
if this is all ok, is openhab user part of dialout group?
sudo adduser openhab dialout
Regarding the repo, see here:
I would suggest to use RC1. This works perfectly for me and I use quite some bindings and rules.
For installing compat1x you need to enter the karaf console (see https://docs.openhab.org/administration/console.html)
and run
feature:install openhab-runtime-compat1x
i hope this helps you further, met plezier gedaan! 