Today we received this from Amazon:
On August 16, 2019, Calibre Companion's integration to the Amazon Drive API will be discontinued. Customers of Amazon storage will continue to have access to their content through our first party applications for Amazon Photos and Amazon Drive. Please make arrangements to discontinue all traffic to the Amazon API before August 16, 2019.
We don't know if all apps are impacted by this decision or whether CC is special.