Outlook add-in deployed to Exchange Server on premises not loaded in macOS Safari

Question

We have a Outlook add-in. When we deploy it to Office 365, everything is fine. All major browsers on Windows and Mac are working.

However, when we deploy the add-in manifest to Exchange Server 2016 on premises, the add-in is not loaded in Safari browser. All major browsers on Windows work, Chrome on Mac also works fine. Just Safari not working - after clicking the add-in icon, the custom task panel opens, but the content is not loaded. It's just blank, no spinner icon.

The JavaScript console shows some errors when the add-in is being opened, such as:

• SyntaxError: Unexpected token '<' for file logon.aspx
• Failed to load resource: the server responded with a status of 400, for file https://mail.xxxxxxx.com/owa/prem/15.1.1591.8/scripts/owa.clientnext.frepanel.js.map

Our Exchange server 2016 has latest CU so far (CU 11 of Oct 2018), and the Safari version is 12.0.2, running on macOS High Sierra version 10.13.6.

Is this a configuration issue on the server or client side? Any suggestions?

Thanks!