I'm trying to use a license purchased previously which is no longer used at a previously made website, which is now gone. So I have no way to "deregister" it.
Found the tool http://api.muffingroup.com/licenses/ to de-register a license from a website that is no longer live. This was the solution.
Great to see that you found it.
If you have any other questions or problems, feel free to ask.