The error is due to the fact that eBay detects unsafe code (HTTP and non HTTPS) between the links of your template. First make sure your e-commerce is using secure mode (SSL) and has a valid certificate installed. If you are using a custom eBay template, carefully check the source code and make sure it does not contain links or links to images, pages or CSS in HTTP format.
If yes, change them to HTTPS. If you have updated the form from a previous version and you are using the {GALLERY_V} tag to show product images dynamically, you need to remove it from the template as it is no longer supported by the new eBay rules.