DMTS_MonikerNotFoundError

When we attempt to GenerateToken using PowerBI API v2 to get hold of an embed token in an RLS/App Owns Data scenario, we are getting back the following response:

{"error":{"code":"DMTS_MonikerNotFoundError","pbi.error":{"code":"DMTS_MonikerNotFoundError","parameters":{},"details":[]}}}

Has anyone dealt with this before and know what could be wrong or how to understand more, i.e., is there somewhere in Power BI where I can see the logs and what is going on here, and what does DMTS stand for?

I pass in a JSON object as below to the `GenerateToken` call:

{
    "accessLevel": "View",
    "datasetId": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
    "identities": [
        {
            "username": "userx@mydomain.com",
            "datasets": [
                "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
            ]
        }
    ]
}

Another observation is that, if I pass in the master account (the user who we connect to Azure as, while embedding) instead of `userx@mydomain.com`, then the same code works fine and we get an embed token.

In the sample app, this outputs the following error:

Operation returned an invalid status code 'NotFound'<r><r> 
at Microsoft.PowerBI.Api.V2.Reports.<GenerateTokenInGroupWithHttpMessagesAsync>d__16.MoveNext()