Broken SPACES API after upgrade

[Albi Gjino]

We upgraded from Confluence 6.13 to 7.9.18 by performing a second installation of Confluence and importing the database. When logging in and trying to access the list of spaces through the "Spaces DIrectory" link it says "An error occurred while searching for spaces."

After debugging the issue, I noticed that the UI is sending an AJAX request to the backend and getting XML as response, but it tries to parse it as JSON and errors out. The  request header is actually for JSON, but the backend returns XML. HEADER ACCEPT is "application/json, text/javascript, */*; q=0.01"

What part of the upgrade process could have caused this, and what can I try to make the backend return JSON?

[Albi Gjino]

It turns out the problem was in AWS CloudFront. CloudFront wasn't forwarding the headers and therefore the ACCEPT header wasn't going to the backend, which was defaulting to XML. I added the headers to the Cache Key in CloudFront Behavior and now it works fine.