Error when updating connection string in Power BI Embedded

I'm using the Power BI Embedded sample application to import a PBIX desktop file into an existing workspace. The import works fine but when I try to update the connection string, I get an error:

Ooops, something broke: Microsoft.Rest.HttpOperationException: Operation returned an invalid status code 'NotFound'
at Microsoft.PowerBI.Api.V1.Gateways.<PatchDatasourceWithHttpMessagesAsync>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.PowerBI.Api.V1.GatewaysExtensions.<PatchDatasourceAsync>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at ProvisionSample.Program.<UpdateConnection>d__28.MoveNext() in C:\Projects\power-bi-embedded-integrate-report-into-web-app-master\ProvisionSample\Program.cs:line 625
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
at ProvisionSample.Program.<Run>d__18.MoveNext() in C:\Projects\power-bi-embedded-integrate-report-into-web-app-master\ProvisionSample\Program.cs:line 240

Any idea what might be the problem?