I'm having the same issue, I'm trying to connect to my Dell RM1200 console port via the onboard Serial connector on my host server.
I'm using Putty app in Unraid, and when I try and connect, it says
unable to open connection to /dev/ttyS0:
Opening serial port '/dev/ttyS0': Permission Denied
Console shows:
root@Unraid3:~# dmesg | grep tty
[ 0.073509] printk: console [tty0] enabled
[ 0.316232] 00:01: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
root@Unraid3:~#