The error:
Your Sitemap or Sitemap index file doesn』t properly declare the namespace.
This error appears when something on your site is modifying URLs after the sitemap is generated. This is virtually impossible for us to locate or fix as it』s not something controlled by our plugin.
However, we have seen a few cases where WP Engine』s HTML post processing feature contains a line (similar to the examples below) that when removed resolves this issue but may break other things on your site. Please contact WP Engine for advice on how to exclude the sitemap namespace from this command.
#http://# => // #http://# => https://