Glad I stumbled across this as I'm observing the same issues today.
I have 9 CSV and 1 XLS file uploaded to One Drive for Business and seeing the invalid credentials message when trying to set up a scheduled refresh using Power BI service.
I then try to edit the credentials (oAuth) which sometimes appears to work okay for the first few data sources, but then I start seeing the same "Credentials can't be updated" message that others are describing above.
Everything works okay in Power BI Desktop, and I've tried deleting everything from the Power BI service and publishing again.
Unfortunately installing the personal gateway isn't an option on this client's site (I have no control over this) so I can't test if that works as suggested above.
Is there any update on this?
Thanks
Pete