The solution here was to change permission for everyone to read/write from /dev/ttyACM0 (i.e from 660 to 666)
sudo chmod 666 /dev/ttyACM0
ll /dev/ttyACM*
crw-rw-rw- 1 root dialout 166, 0 Jan 28 23:37 /dev/ttyACM0
p.s.
I first tried to add user openhab to group dialout (or to group tty) but this didn’t help.
Maybe the user from eclipse is no openhab?
sudo usermod -a -G tty openhab
sudo usermod -a -G dialout openhab