I am trying to create a publishing page (with a system-provided page layout, Article) in SharePoint 2013's SitePages document library. SharePoint shows the create page screen, where it asks for the page layout and file name, but shows the wrong document library: always Pages instead of SitePages, even if I came from SitePages, and it actually shows in the URL; I cannot change it because it is a text literal, not a selector control. Then, when I click create, I get an exception about an invalid value. All of the publishing features are active, the document library has the right content types and they show up in the add file option. ULS logs just show the exception, no relevant information. What could be wrong?