Did you enable by chance the DHCP within the pi-hole ?
A DHCP server failure is most likely due to a config error (a wrong parameter saved in one of the config files). Upon restart, the dhcp server doesn't start because of the said config error.
Maybe the OWRT’s web interface is writing the config with a bad string.
What you can do to try the Pi-hole blocking is to run a ping on a known blocked domain (like flurry.com) from the device "hosting" pi-hole.
You can also try to set the Pi-hole IP as your sole DNS on one of your clients.
Test it like that.
If it works, you isolated the problem to the router.
What you could do in this case is disable your router's LAN DHCP and use Pi-hole as your DHCP server.
That way, the DNS setting and hence the ad-blocking, will be pushed automatically to all your connecting clients, removing the need of you actually having to set the DNS manually on them.