Solved.
In my wp_config, I’ve definded WP_PLUGIN_DIR. If you have this too, you have to change a few things…
With WP_PLUGIN_DIR plugins are not called via https://example.com/wp-content/plugins/shariff-sharing but via https://WP_PLUGIN_DIR/shariff-sharing.
In /shariff-sharing/Backend/ I had to edit the index.php:
1.
Add header('Access-Control-Allow-Origin: *'); just above or below (it does not matter where) the other header(...).
You may change * to the real WordPress-URL (https://example.com) for extra security.
2.
Search for "domain" => $_SERVER["HTTP_HOST"] and replace $_SERVER[“HTTP_HOST”] with the your real WordPress URL (e.g. “example.com”, without http/https).
Maybe Plugin-Author can change this to the constant for the current WP-Site-URL…
3.
Make sure the path at cacheDir exists and is writeable. On my shared host it was not I had to change it.
Maybe Plugin-Author can change this from /tmp to a subdirectory within wp-content (and create this directory on plugin-activation).
