Posting this as I opened a Support Case to review the error and because this may assist others as I expect use of Xtract to increase.
After starting to write this thread i saw there was a new version (today!) so I upgraded to 1.1.3. I decided to reboot the VM as well. The upgrade was simple, I decided to log in to the Xtract VM with the default credentials in the Admin Guide and reboot the VM. I couldn't connect to the web GUI, so did an ifconfig and didn't see my inet (IPv4) address, so rebooted again and it showed up. Connected back into the web GUI, restarted the migration and noticed a nice "new" thing with a hour(s) minute(s) remaining along with the percentage progress indicator! So it's back running now with roughly 5 hours to go (moving over 1Gbps network).
TL;DR - "Reader is unavailable" error, try restarting the Xtract VM (ensure your jobs are paused).
Best answer by BG_selfcloud
"From what I am seeing in the logs it appears that the Xtract diskreader lost connection with the source and was not able to reconnect.
This same signature (shown below) has been seen in other cases caused by duplicate IPs, network disruptions, and other network related issue."
So I chocked it up to some network issue and rebooting the Xtract appliance allowed the pathing to fully re-establish.