error undefined

I have been this mistake for some time I have attached a small video.
In practice I realized that it happens perhaps more often after canceling lines because maybe there are double products.
After except for the changes during the rescue, this error takes place. The wheel turns eternally does not stop. I have to click OK and restart the page. But some changes don’t save them. I have to redo the work. Could you help me?
I write as administrator of the website http://www.love4music.it with my partner Giannibella who purchased the product.
video link