You may be using the wrong URL.

Sorry, typo in my post, not in what I entered in Terminal. Checked again, and I did use the //.

Can you ping "plexserver". Does it resolve to 192.168.0.31?

Yes, and Yes.

ping 192.168.0.31
PING 192.168.0.31 (192.168.0.31): 56 data bytes
64 bytes from 192.168.0.31: icmp_seq=0 ttl=64 time=0.676 ms
64 bytes from 192.168.0.31: icmp_seq=1 ttl=64 time=0.652 ms
64 bytes from 192.168.0.31: icmp_seq=2 ttl=64 time=0.454 ms
64 bytes from 192.168.0.31: icmp_seq=3 ttl=64 time=0.702 ms
64 bytes from 192.168.0.31: icmp_seq=4 ttl=64 time=0.603 ms
^C
--- 192.168.0.31 ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
[/code]

Is ClearOS in Standalone or Gateway mode? Is it doing DNS/DHCP?

Standalone, no firewall. It is not doing DNS. It WAS doing a (duplicate) DHCP server by mistake, but I changed DHCP to Disabled, restarted router (which is the DHCP server), ClearOS and my iMac.

It makes no difference.

FWIW 192.168.0.0/24 is not a brilliant choice of subnets.

As it is the router's default, I just used it.

What's the drawback?

It would be a bit of a pain to change it, as I have about 30 permanently active devices on the network, and another 20 that were or can be active occasionally.

Most have reserved fixed addresses handed out by the router.