Plugin does not work with file upload

Hello, I want to give customers on my site an option to upload a file and add a short note using the Product Add-Ons plugin. The file must always be uploaded by a customer, the note is optional. If the file and note are added successfully, they are visible in the cart.

However, if I enable your plugin (which I like very much), the product does not get added to the cart. Instead, a weird “battery”-like image appears at the bottom center of the screen. Do you know what the problem could be? I would really like to use both plugins and purchase an upgrade of your cart plugin.

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