“Failed to load content css” since WP 4.8

Hi there,

since I updated to WP 4.8 I get this error message when entering the editor from a page or post: “Failed to load content css: http://mywebsite/wp-content/plugins/foogallery/css/admin-tinymce.css“. It is a mixed active content error according to the console in firefox. Everything is working fine but this message comes up everytime. I tried to fix it with the plugin “SSL Insecure Content Fixer” which is running all the time on my website to prevent these mixed content issues. But it didn’t helped.

The function plugin_dir_url delivieres the correct url with https://.

We are forcing https with NGinx reverse proxy for every connection via port 80 like this: location / {
return 307 https://$host$request_uri;
}

Also to avoid mixed content issues we are using this plugin

We found something in the core of WP but didn’t dig in further:

--- wp-includes/link-template.php.orig2017-06-14 12:04:53.000000000 +0200
+++ wp-includes/link-template.php2017-06-14 12:04:58.000000000 +0200
@@ -3212,7 +3212,7 @@
 $url = WP_PLUGIN_URL;
 
 
-$url = set_url_scheme( $url );
+//$url = set_url_scheme( $url );
 
 if ( !empty($plugin) && is_string($plugin) ) {
 $folder = dirname(plugin_basename($plugin));

Before the set_url_scheme the url contains https after it only http.

By commenting it out it works without any mixed content issues but changing the core of WP feels not like a solution.

I already posted this in the specific plugin forum and got contact with the plugin author but he didn’t response properly so I’m trying it here. Old topic