To document what I think happened. I think the project was pre-registered some months ago, from those lists of projects that were collected by different areas. The project was registered in Paratext 8, with a certain set of team members. At migration time, the team asked someone else to migrate the project for them, and added them as an admin to the Paratext 7 project. The new admin starts the migration, Paratext 8 says the project is not registered, because he is not included in the existing registration. He submits the registration info, which is rejected because the project is already registered.
The solution, find the existing project registration on the registry server, go to members, and click the “authorize user for migration” link to add the new admin into the existing project.