That looks like DNS rebind protection in action if OpenWRT is responding.
I’m guessing your DNS is pointed at your router’s external IP address and you’re getting this error from a host inside your router’s LAN.
If that’s the case, you probably don’t want to disable DNS rebind protection, so you’ll want to look into enabling hairpin NAT instead so that OpenWRT forwards the internal → external → internal packets properly according to port forwards. I think on OpenWRT they refer to it as “loopback NAT”.