I'm not an Arch Linux user (Ubuntu here), but since nobody has answered...
Have you checked the rest of the serial ports?
cd /dev
ls tty* -l
See what groups those have assigned. Mine (ttySxx) are all dialout group. Many Linux users here have problems if they do not add the dialout group to your user. You might try adding that.
Use this to check what groups your user belongs to.
groups
If you feel you want to add the dialout group to your user
sudo usermod -aG dialout yourusername
You will need to logout and login for the group change to take effect.
If that doesn't do it, maybe someone else will see the problem.