Device ownership and permissions? ls -l /dev/ttyS*
To allow ANYONE to write to the serial ports (as root):
chmod 666 /dev/ttyS*
Alternatively, check the group ownership and permission. If it's something like "uucp" then add the uucp group to your list of allowed groups.
usermod -G <current groups>,uucp <your user id>
Also check if udev manages the devices. You may need to update the udev rules to make any changes permanent.