The image above posted for the page layout in SharePointDesigner looks like this
in the question is showing two content editors on page.
Remove the the webpartzone which is outside of the PlaceholderMain ContentPlaceHolder (highlighted in red in the snapshot i have attached).
Old post, but answering to help those who may come into the same issue. I had the same issue. Created a new site column of Publishing HTML format and wanted to add another area on a page/page layout that people could add more text/HTML/or web parts.
Received same errors as above.
It seems that you MUST put these fields inside the PlaceholderMain area.