there is an unattractive bug when mofifing a Data source from Report when you have added a Scheduled refresh plan I'd like you to know.
When you create a Report with e.g. Excel Data Sources and you insert credentials within Report Server Portal and add a Scheduled Refresh Plans ("Every 5 Minutes" and "Every 02:00 AM)" you are fine until you modify Data Sources from Power BI Client. E.g. you want to change Data Source connection string from NetBios name (\\servername\share\Excel1.xlsx) to FQDN (\\servername.domain.com\share\Excel1.xlsx) and save changes to Report you will notice that you'll need to re-enter Data Source credentials again. But Scheduled Refresh Plan seems to be gone also.
At this point of time you will notice that within Report Server Logs you will see that Sheduled Refresh Plan, which isn't visible that the moment because of missing credential update is still going to be executed.
I've searched within Report Server Database and by checking tabled "dbo.Catalog" for Report ID, "dbo.Schedule" for ScheduleID and "dbo.ReportSchedule" for result you'll see that Scheduled Refresh plans are still added to Report even if there is no hint from Portal about it. Therefore no credentials are added to connection to Excel file you'll notice that at each runtime an error will be shown in Report Server Refresh logs that there are missing / false credentials are added to Scheduled Refresh plan's data source.
Once I re-add credentials again, both plans will appear again.
I'd like to let you know about this behaviour, maybe you can implement some kind of note to report's Scheduled Refresh plan when a refresh plan is added to report but data source was updated and you need to enter credentials again.
For Portal user it looks like there is no scheduled refresh plan added to report under this condition.
This will also happen, if you add new connections to report also.