I am also new to using unraid and I've actually run into the same issue.. ran into your post while searching for information... so far I narrowed it down to being a combination of using the prebuilt CentOS option in unraid and using the bridge network "br0" when building the vm, once you activate the network in the installer, if you go in and out of the installation destination menu it crashes with that unknown error. (Instead of br0 I changed it to virbr0 in the vm settings and it worked fine, but I didn't start the install, I only attempted to reproduce that error.. this wouldn't work for me cause I wanted it to be visible to my main network)
I am on unraid 6.6.7 and using the CentOS-7-x86_64-DVD-1810.iso
My way around it was to build the vm using the Linux option instead of CentOS in unraid, from there it allowed me to install centos, hope this helps!