In a book with 14 pages, I right-click on a page in multi-page view and select "Add Page". The following pop-up error appears. LR CC version 2015.12 release, Windows 10 Pro version 1703. Today the behavior has changed. Yesterday NO new page was created after I clicked OK on the pop-up error window. Today it creates a new blank page, same as the "Add Blank Page" command. I am using a custom page layout with one image and one photo text field that contains the image caption and the image's capture date and time. After I create a new blank page I can apply my custom layout and it all appears to work properly. If I right-click on the new page with the custom layout applied and select "Add Page", the same error message appears.