Posts are now back in business. It was the the WordPress SEO-plugin that caused the problem. Or at least the links to the posts started working again when I deactivated the WordPress SEO-plugin. (Before, no matter if I clicked on a link in a RSS-reader or wrote the URL I ended up either on a 404-page or compleatly wrong).
However, the news/blog page still only show 2 excerp, not 4 full posts before the grid.