Office API call to getUserIdentityTokenAsync() returns a null token

Question

Our Outlook add-in runs successfully for our production clients except for one specific tenant.

With this one tenant, the Office API call to getUserIdentityTokenAsync() returns a null token.

Here is what I have determined from reviewing our logs:

1. When they access the addin from the Mac Outlook client, getUserIdentityTokenAsync() returns "Status" = "failed" with error code "9017".

2. When they access the addin from Chrome or Firefox, getUserIdentityTokenAsync() returns "Status" = "succeeded" but the token value is null.

Their self-hosted Microsoft Exchange server version is 15.1.1531.3 and I have confirmed Exchange Web Services is enabled.

Does anyone have insight as to why this method is failing to return a token?