if this “An error occurred during the processing of . Only Content controls are allowed directly in a content page that contains Content controls.” error message happens to you, let me tel you the solution can be very easy.
Ususally this is associated with MasterPages or PageLayouts, so check in you pages if there are any content tags with lowercase, and just replace them with Content! that ‘s it!!!
content –> Content
It is a very stupid error, but it happens.