Invitations: Person invitation throws unexpected error

I created a person, clicked "Invite person"(, logged out of AlekSIS) and opened the link provided in the e-mail. When I enter a username and password and click on "Sign up", the following error is shown: Person with this Site and Short name already exists.

grep -ri "Person with this Site" in AlekSIS-Core/ and django-invitations returns no result.