Just discovered!
If you have a forum running under https and you attempt to move a forum using the "Change Parent" and enter the full URL as instructed, you are informed the URL is invalid.
The current work around is to delete the "s" from "https://"
Hopefully this is an easy fix for the Nabble team!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.