Hi Tony
I haven't seen such a thing. Can you pls upload the log bundle of the failed task or send it to me in a private message?
Do I understand correctly that the large volume is a simple volume at the destination?
You may try to put the large volume in its own volume group (Converter creates a separate LVM volume group per destination virtual disk) and keep the LVM. That's just a wild idea, I don't know whether it will work but it's an interesting experiment (you may try it with a thin provisioned disk lest the datastore would be insufficient).
Regards
Plamen