I ende op getting the problem solved in this thread on the Plex forum: https://forums.plex.tv/discuss…-signing-in-to-plex?new=1
Apparently when I changed the DNS in the web client it did not work but when I changed it here "/etc/resolv.conf" it did work.
So it was a DNS problem.
Thanks for the help 
static ip was also the plan I just hit this problem before I got to set up static ip.
I dont think it is my router when my other 2 computer have no problem with DHCP and my friend with another router and internet have the same problem.
We did the exact same thing when we installed OMW and Plex. So I think we did the same thing wrong to.
Edit: I had some kind of bug that made me post 3 times.
omv-showkey network
2.1.23 64 bit
I can also say that I made this server together with a friend(1 each) and we both have the same problem even though we are at to different places/networks.
Edit:
The only thing we have installed is Plex.
With DNS from DHCP
cat /etc/network/interfaces
The loopback network interface
auto lo
iface lo inet loopback
# eth0 network interface
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
iface eth0 inet6 manual
pre-down ip -6 addr flush dev $IFACE
cat /etc/resolv.conf
search local
nameserver 192.168.0.1
With Manual DNS
cat /etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback
# eth0 network interface
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
dns-nameservers 8.8.8.8
iface eth0 inet6 manual
pre-down ip -6 addr flush dev $IFACE
cat /etc/resolv.conf
search local
nameserver 192.168.0.1
I know that 8.8.8.8 doesn't use DNS. It was just to show that it probably is a DNS problem
Yes I used the web interface to change the DNS.
Also shouldn't it work with the router DNS from DHCP?
When I try to ping google.com I get this: ping: unknown host google.com.
When I ping 8.8.8.8 then there is no problem.
I have tried manually setting the interface dns to 8.8.8.8 but that doesn't change anything.
OMW is new to me so it can be something stupid I have overlooked.
hope someone can help
Also a side question: The firewall has a default allow all rule if I have not made any rules myself right?
