Gutenberg “Preview Button” Not Loading – “Generating Preview…”

Hello

We’re having an issue with the “Preview” button on Gutenberg just stuck “Generating Preview…”

See video clip here: https://i.gyazo.com/75e6d3c723cf305002c5e7c81d87ec7b.mp4

The website is using an “IIS reverse proxy”.

I’ve looked at a few posts etc on here and Github, that shared some suggestions/fixes, mainly caused by Cloudflare reverse proxy, however none of these solved it.

The main suggestions are:

(1) Clear the reverse proxy cache.
The cache on our IIS reverse proxy is cleared automatically every 5 minutes.

(2) Make sure the server IP and your own IP addresses are whitelisted.
“We don’t blacklist/whitelist anything on the reverse proxy so there is no whitelist to add to.”

It is also not a plugin/theme conflict, since a duplicate of the site onto a staging server, without the reverse proxy works fine.

Does anyone have any other suggestions to fix this?

Cheers!