Same here catsknit. I had no idea what the problem was until I checked the more info button to find the too many requests error. At first I had no idea what was going on, just that every single book came back with no metadata results.
Even when I use a VPN I can only grab metadata for about 4 books, then I get blocked by Google and no metadata gets returned for anything. If I close Calibre and switch IPs I can get metadata for 4 more books before being blocked again.
This isn't even large batch downloads, just 4 separate books individually. I don't know if Google changed something or if it's Calibre but it never happened before, even when grabbing metadata for dozens of books.
