Hi,
This indicates that you have a fatal error during the processing of the page.
Please activate the "debug mode" and "error reporting" settings of the Joomla configuration and go to that page again. Then, scroll to the bottom, and you should see the error message which will indicate the file and line where the problem happened. We need to know that error message to be able to tell you what to do.
Since it's only on the product page and category page, it's probably linked to a problem with your HTML editor, that you might need to update or use another one.