@wlp94114 I had the same error. My ISO repository is on an SMB share, but the files were not readable by the ID I had mounted the share with in Xen. I corrected the permission on the file, and could get the VM to start.
Thanks for the feedback. If you couldn't find this in our troubleshooting section of our doc, feel free to contribute, that will be helpful to others too 🙂