CORS error on image fetch request

Hello,

we have a missing CORS header error from the Crop Thumbnails plugin in one of our clients site. The problem started after we made some infrastructure changes and split the site to two servers.
The server configuration has two servers both Ngnix. One server listens to http://www.example.com while the other listens to custom.example.com . The custom prefix serves authors, while www serves all non-logged users. Both servers replicate the site and have synced databases.

When an author tries to crop an image using the plugin from the media gallery (either list or grid view), the modal loads, the different sizes of the image load on the left side of the modal, but the RAW image on the right side stays loading and the request that fetches the image resource produces a missing CORS error and has status 200. The stack trace on the request is as follows:

B1/< wp-content/plugins/crop-thumbnails/app/main.js:70:30428 B1 wp-content/plugins/crop-thumbnails/app/main.js:70:29569 onChangeImage/i< wp-content/plugins/crop-thumbnails/app/main.js:71:28328 onChangeImage wp-content/plugins/crop-thumbnails/app/main.js:71:28302 mounted wp-content/plugins/crop-thumbnails/app/main.js:71:17161 mi/
wp-content/plugins/crop-thumbnails/app/main.js:13:15956
dispatch
wp-content/plugins/crop-thumbnails/app/main.js:10:40223
add/Pe.handle
wp-content/plugins/crop-thumbnails/app/main.js:10:38158
(async: EventListener.handleEvent) add
wp-content/plugins/crop-thumbnails/app/main.js:10:38638
Ya/< wp-content/plugins/crop-thumbnails/app/main.js:10:37785 each wp-content/plugins/crop-thumbnails/app/main.js:10:2799 each wp-content/plugins/crop-thumbnails/app/main.js:10:1275 Ya wp-content/plugins/crop-thumbnails/app/main.js:10:37761 on wp-content/plugins/crop-thumbnails/app/main.js:10:45181
wp-content/plugins/crop-thumbnails/app/main.js:13:15707
Pe
wp-content/plugins/crop-thumbnails/app/main.js:10:27620
Deferred/then/k/</xt<
wp-content/plugins/crop-thumbnails/app/main.js:10:27954
(async: setTimeout handler) Deferred/then/k/<
wp-content/plugins/crop-thumbnails/app/main.js:10:28223
E
wp-content/plugins/crop-thumbnails/app/main.js:10:25677
fireWith
wp-content/plugins/crop-thumbnails/app/main.js:10:26442
fire
wp-content/plugins/crop-thumbnails/app/main.js:10:26478
E
wp-content/plugins/crop-thumbnails/app/main.js:10:25677
fireWith
wp-content/plugins/crop-thumbnails/app/main.js:10:26442
ready
wp-content/plugins/crop-thumbnails/app/main.js:10:29628

It seems that the request although generated by custom.example.com it has its origin set as http://www.example.com .
We even tried adding access-control-allow: * header via Ngnix, but while all other requests get this header, the specific request that fetches the image does not.
We have really run out of ideas how to solve this and would appreciate any help! Thank you for your time!