Why does it say "email address already in use" when no password has been set?

A colleague of mine tried to create a developer account but something went wrong. He went through the signup steps and at the last step when he confirmed his details, an error popped up. Now when he tries to signup again he receives an error saying his email address is already in use but there is no way to retrieve it because no password has been set.

The email address used is m.vdlinden@loyally.eu.