Looks like you are missing one letter! From the documentation on https://developer.gracenote.com/web-api, the correct endpoint should be:
https://cXXXXXXX.web.cddbp.net/webapi/xml/1.0/
Notice the "p" at the end of the "cddb".
Also, make sure that you replace the XXXXXX's with only the digits that precede the hyphen in your Client ID string. i.e. if your Client ID string is "1234567-09876543210987654321" (not a real Client ID), then you would send requests to https://c1234567.web.cddbp.net/webapi/xml/1.0/