Sebastian says that Amalie is "an alumni" of the summer camp. "Alumni" is plural (either solely males, mixed gender, or gender unknown). "Alumnus" would be correct for a someone whose gender is unknown, but "alumna" would have been most correct, as it is known that Amalie was female.
When Sebastian looks up Stefan's information, his last name is Hoster, while in the credits the character's name Stefan Vogel.