Why does 'Add to Cart' cause an error after photo upload?

Hi, I have a new store that sells custom products.
The customer should upload about 10 photos on the product page and then add to the cart and make a purchase.

For uploading the images on the product page I added the “Personalized Image Uploader” app.

What happens now on the product page is that as soon as you add the photos and click Add to Cart you get an error “something went wrong”.

I talked to the app support and they did not find any problem…

I will ask for urgent attention on the subject!