It looks like its Mindsphere returning the 403 error, so I would ask whoever runs the API to check if they have it setup to allow CORS from your domain. If they haven’t, you’ll see CORS errors as you previously mentioned.
The first idea I have is, that you have not grant your application access to the MindSphere API you are calling.
You can do this in the MindSphere Developer Cockpit.
Please select there your application and assign here the needed MindSphere API role
Hope this is helping.
Stefan