WordPress showing custom post type instead of 'post' on single.php

For some reason, WordPress has started showing the custom post type (nyhet) that I have added instead of the standard post type on single.php. When I click on http://localhost/demosajt/punk-magazine-nummer-13/, which links to a standard post, instead it shows a “nyhet” post type post http://localhost/demosajt/nyhet/det-har-ar-en-till-nyhet/.

It’s hard to explain, but essentially, it has replaced all standard posts with the custom post type nyhet. This doesn’t happen when I delete all posts of the nyhet post type; then it works as intended.