I do not have that plugin — “WC Fields Factory” — and this is not an issue with other plugins acting as a conflict.
That message is generated from the WooCommerce Wishlist plugin itself.
I’ve used a plugin by the name of, “String Locator” to find this message, and it came up as a message buried within your plugin. This message in your plugin is located on lines 298 and lines 413. In the “wp-content/plugins/ti-woocommerce-wishlist/public/wishlist/buttons.class.php” file.
The only time I see it come up is when I choose “Add Selected to Cart” and that item is already IN the cart. Otherwise it does not appear. I’m trying to “break” my site and do things that are “forbidden” by plugins to test what happens, and that is when I saw this message come up.
Granted, it’s only appearing when a user tries to add a product to the cart that is already IN the cart, but still. It’s confusing as hell, and it isn’t even an appropriate message for my product type. They are all single sale items.
So no, this is not an issue that is, as you first said, “The issue may appear if you are using some additional Woo add-ons. There are hundreds of WooCommerce addons on the web that allow to use custom meta data. Unfortunately there is no global solution to make our wishlist compatible with all of the other plugins. In most cases it requires integration of these plugins from our end.”
And if you DO support “all WooCommerce types” as you stated, then you should be able to support single sale items. There are no additional “options” for these items, so having the plugin tell customers they need to select additional items is not correct — so that message should never appear.
Please fix this in your next update. Pretty please. Thank you.