I'm having problems with the DatePicker in SP2013. The control shows fine, but the calendar popup is broken because of a missing stylesheet (the complete sprite sheet image is visible etc.).
The css missing is corev15app.css. It should have been returned by the _layouts/15/defaultcss.ashx handler called in the calendar iframe source, however this fails. Here is the related log output:
defaultcss.ashx: resource specified null defaultcss.ashx: attempting to get web.MasterCssUrl defaultcss.ashx: using elevated codepath to get css file or other resource because the non-elevated code path failed to get it. (...) defaultcss.ashx: attempting to get web.MasterCssUrl Application error when access /_layouts/15/defaultcss.ashx, Error=The object specified does not belong to a list.
Additional info: I upgraded a SharePoint 2010 solution, set up new layout pages and used the new seattle master page from SP2013. Just about everything works as it should. However the DatePicker and the "Add an app" page (addanapp.aspx) fails. The app page just stops with the loading animation.