"Unable to save" message bug save works

I'm most likely returning something wrong but couldn't find it in the docs.

When using the Zoho Writer Office Integrator API and saving a .docx file, it gets posted and saved correctly to the save_url but the UI shows:

"Unable to save: An error occurred while posting the content to your storage server. Please try again."

What's the request expecting as a response?, I'm returning a HTTP 200 with "RESPONSE: Document saved" as content.

Tried without content and it's giving the same message.

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