Had the same problem today, caused by a update of the (external) server API which changed its CORS settings.
Looking at the network tab of the developer tools in the browser made it seem like everything was OK (200), only in the console it showed the problem.
Further reads which helped me find the problem:
CORS - Is it a client-side thing, a server-side thing, or a transport level thing?
How to solve 'Redirect has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header'?