Hi experts,
I've come across an odd issue when trying to preview excel attachments anywhere in the system (Sales Orders, Inventory Journals, etc). The preview refuses to load and instead throws a "Service Unavailable error", pictured here.

Here's some info:
No customization has been made to these screens, I've tried with multiple users and browsers, clearing cache and whatnot.
This happens in Dev VMs (I've read this is intentional by Microsoft, but I'd like to know if there's a way to go about it), and in our Sandbox and Production environments. The Sandbox and Production environments are running these versions:

Going to document management parameters, I've set the value for "natively web renderable" to yes for both of the excel types, but "No" and "Yes" don't really change the results.

We do not have a sharepoint server configured, but I don't know if this is relevant to the issue.
Finally, the Office Web Apps Server is configured to https://onenote.officeapps.live.com, as per Microsoft's documentation.

Is there a configuration I'm missing, or an update that solves this issue?
Thanks in advance for any insight you can give me here!