PowerBI Embedded not accepting connections using service principal and security secret.

We are connecting to PowerBi Embedded using stored credentials (service principal with a secret.).  We are using this via an Azure .net core IIS service to embedd iframes in our website.

This has been working fine for several weeks, but stopped working yesterday. It appears the secret is not being accepted.

We also have a development instance trying to connect to the same reports. (The reports run fine in the PowerBI portal)

We've enabled debugging and are receiving the following error.

Operation returned an invalid status code 'InternalServerError'

at Microsoft.PowerBI.Api.ReportsOperations.GetReportInGroupWithHttpMessagesAsync(Guid groupId, Guid reportId, Dictionary`2 customHeaders, CancellationToken cancellationToken)

at Microsoft.PowerBI.Api.ReportsOperationsExtensions.GetReportInGroupAsync(IReportsOperations operations, Guid groupId, Guid reportId, CancellationToken cancellationToken)

at Microsoft.PowerBI.Api.ReportsOperationsExtensions.GetReportInGroup(IReportsOperations operations, Guid groupId, Guid reportId)

at AppOwnsData.Services.PbiEmbedService.GetEmbedParams(Guid workspaceId, Guid reportId, Guid additionalDatasetId) in C:\Temp\PowerBI-Developer-Samples-master\PowerBI-Developer-Samples-master\.NET Core\Embed for your customers\AppOwnsData\Services\PbiEmbedService.cs:line 46

at AppOwnsData.Controllers.EmbedInfoController.GetEmbedInfo() in C:\Temp\PowerBI-Developer-Samples-master\PowerBI-Developer-Samples-master\.NET Core\Embed for your customers\AppOwnsData\Controllers\EmbedInfoController.cs:line 45

We've deleted and recreated the service principal and redeployed the IIS service but it still won't work.

What have we missed? Any advice or suggestions would be greatly appreciated.

Pete

Edit: We are hosted in Australia East