This is caused by a secure reusable page that is called into the non-secure Pagebuilder page.  To check for this:
  1. Navigate to Content > select PageBuilder.
  2. Click Edit Content under Actions for the page in question.
  3. If you are not already using the Plain Text Editor, click the pencil icon in the top right corner to view the HTML source code.
  4. Identify any embedded content or reusable pages included in the content of the page.  The easiest way to do this is to locate the tag <convio:session name="51" param="page_name"/> , where the parameter is the name of the PageBuilder page.
  5. Locate the referenced PageBuilder page(s) in the PageBuilder Library and click Edit Attributes.
  6. Navigate to 7. Page Properties and unmark the "This is a Secure Page that will be encrypted via SSL" checkbox.