Can't schedule a report refresh in PBI Service

[cymorg]

I have a report with 2 tables sourced from separate web API calls to a similar service, let's call it MyService.  The service returns json.  The report refreshes perfectly within PBI Desktop.  The report publishes to PowerBI.com where it runs without a hitch.  On-demand refresh of the dataset in the PBI Service works perfectly too.

I can't schedule a refresh for this report because the option is greyed out.  There is an error reported in the Data source credentials section where one of the two Web sources reports it cannot connect to MyService.  The error is... "Your data source can't be refreshed because the credentials are invalid. Please update your credentials and try again." Attempting to edit the credentials for the failing connection results in a 500 Internal Server Error.  

This error is unexpected because I understood the on-demand refreshes & scheduled refreshes would use the same data source(s).  There is no where to specify different data sources/credentials for on-demand vs. scheduled so I just assume they share the same sets of credentials.

Is something weird going on or does my understanding of the innards of the PBI Service need realignment?

[cymorg · Accepted solution]

Resolved via workaround...

PROBLEM

Dataset credentials cannot be updated without causing 500 error. Invalid credentials disables scheduled refresh options. When publishing a .pbix file via PBI Desktop to PBI Service it may fail to update the dataset connection in the service.

WORKAROUND

Open the same .pbix file via PBI Service (i.e. PowerBI.com --> GetData), and the dataset connection will be updated. Credentials can now be set without error, thus allowing scheduled refreshes to be set.