I just tested with netstat, and it appear port 53 is already services by kresd, so also setting DNS server port to 53 is bound to give problems:
:~# netstat -tulpn
Active Internet connections (only servers)
Preformatted text`Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:53 0.0.0.0:* LISTEN 3247/kresd
tcp 0 0 :::53 :::* LISTEN 3247/kresd
udp 0 0 0.0.0.0:53 0.0.0.0:* 3247/kresd
udp 0 0 :::53 :::* 3247/kresd
udp 0 0 0.0.0.0:67 0.0.0.0:* 23924/dnsmasq
Last in this list is also dnsmasq, which is the tool which should be responsible for the DNS and DHCP section in LuCi. I find nothing running on port 0, which in understandable given the explanation of the usage of port 0:
Port 0 is a reserved port in TCP/IP networking, meaning that it should not be used in TCP or UDP messages. However, port 0 carries special significance in network programming, particularly Unix socket programming: for requesting system-allocated, dynamic ports. (lifewire.com)
So I find it safe to assume the running instance of dnsmasq on port 67 was the one destined to go to port 53.
So obviously during startup, there must be some race conflict as to which process get port 53. And the bit question is, why is kreds even running? Can it just be shut off?
Well, attempting to check up on the forum, just made this more confusing;
How-to-configure-local-address-dns-resoultion-on-omnia and also Dnsmasq-lan-domain-while-still-using-knot-resolver explains a lot, and on several points looks resolved - untill you read the next comment. 
So what is the right way of dealing with this???