[Flask] CORS issue

[Larry Martell]

I have a flask app that is deployed on many AWS hosts. All of them
work, except for 1, which gets this error:

Access to XMLHttpRequest at 'https://apidse-foo.com/group/1011/arrays'
from origin 'https://dse-foo.com' has been blocked by CORS policy: No
'Access-Control-Allow-Origin' header is present on the requested
resource.

All are running Amazon Linux AMI 2018.03 and all are running the same
flask code and JS code. What could be different on this 1 system from
the others that is causing this? How can I debug this? I added this:

logging.getLogger('flask_cors').level = logging.DEBUG

But I did not get any debugging logging.