Is this the language code entered on the line that reads: 2 Letter language code to search for? I tried 'all' in there and the generated URL started with
https://www.kobo.com/ca/all/search? where all is where a 2 letter language code should be found. It's possible that Kobo updated their web server code to only allow valid language codes in that position.
I just checked back and your search URL started with
https://www.kobo.com/us/all/search? which again is not a valid 2 letter language code
Edit: In a just to see what happens mode, I changed the language code to blank and Kobo Metadata was able to download metadata from a search URL starting with:
https://www.kobo.com/ca//search?. The // was replaced by /en/ by Kobo's end.