How to connect/read/write to serial port without root

Question

As in the title, plus I'm already in group dialout via:

sudo usermod -a -G dialout ben

(I restarted my PC several times since then...)

I'm using Ubuntu 16.04

File permission of the device-file:

crw-rw---- 1 root dialout 216, 0 Aug 26 22:50 /dev/rfcomm0

when I try to run:minicom or putty (as ben, not as root) I got following error:

minicom: Error opening the file »/dev/rfcomm0«: The device or ressource is busy

(same with putty)

But when I do: sudo minicom, everything is OK and works well.
(Device seems not to be busy anymore, when using sudo)

Question:
How to connect/read/write to serial port without root?

Answer 1

This is an answer to my own question.
To solve the problem, I had to remove the modemmanager via:

sudo apt remove modemmanager

After a reboot, minicom (and putty) works without root!

Of course, you also need to be in the dialout group:

sudo adduser $USER dialout

I found the solution in this Ask Ubuntu answer and this Launchpad bug report.