Today I got the following error, when I wanted to open and edit a page based on a custom page layout: Only Content controls are allowed directly in a content page that contains Content controls.
I found a lot of blog entries about that topic, and thanks to this one I finally found the solution:
The cause of the problem is a improper case for the control tags the page layout. Some of the contorls in my page layout used a lower case “c” instead of an uppercase “C”. I just changed <asp:content>
in the whole document and afterwards all the pages based on that layout worked again.