Hi @martinrp,
Im not sure if you have the same issue as me. But I get “sites/0/” in my attachment URLs. If you have the same issue, then use following code in your functions.php.
add_filter('wp_get_attachment_image_src', 'acf_multisite_upload_fix', 10, 4);
function acf_multisite_upload_fix($image, $attachment_id, $size, $icon){
global $current_blog;
if(isset($image[0])){
$replace = $current_blog->blog_id > 1 ? 'sites/' . $current_blog->blog_id . '/' : '';
$image[0] = str_replace('sites/0/', $replace, $image[0]);
}
return $image;
}