Well, it started after I installed the plugin and, depending on it being installed or not, I get different errors. It might be because our post permalinks have a different structure (using /2011/01/posttitle) than the pages (which are directly added after the url /posttitle ). I am also admin at a second website that had the same problem, even with posts (but there they have the same structure as the pages /posttitle). But that page uses PHP 5.2 and after deleting and reinstalling Organize Series everything was working again. It looks like the plugin, trying to use permalinks like /postseries messed up something in the database.
