I to use Unifi products, including the Dream Machine Pro and do not experience this problem. Four different VLAN's and everything works as intended. I also have Pi-Hole running in UnRaid docker container, but do not have any custom DNS records for it.
What is under the "Details" section of the error page that you have there?
A couple of questions though...
What would be the preferred Host or Bridge?
URL, what should it be set to? I have the IP address of my unRaid server. Should it include the port or not? I have it showing correctly when I open it via a web browser, but am having troubles with the uploader finding it.
Thanks again and sorry for the qustions!