Thanks all for your tips!
In the meantime, I've found a method that works and no longer need the "add from URL" option.
- 90% of the books I mean are from orders, either Amazon or other online sellers.
- in Calibre, I open the "Add from ISBN"
- I go back to the page of the book on the sellers page (which I would have to do in the "add from URL" version as well)
- copy the ISBN rather than the URL
- add it to my "add from ISBN" list
and voila, the book is not only added but Calibre finds the precise metadata for the book!
So thanks again, I will keep you tips for those few books that I actually buy in stores