SharePoint credentials invalid - only in Service

In Power BI Service, my SharePoint credentials suddenly seem to fail when refreshing a dataset, for no apparent reason

• This happens with multiple SharePoint sites
• I can acccess all the sites directly (I'm an admin for them)
• I can connect to the sites in Power BI Desktop to create and refresh a dataset connected to an Excel file in SharePoint
• The invalid credentials issue only appears in Service when I refresh
• Everything works for colleagues
  • If I follow the same process to create, publish and refresh the dataset under a colleague's login (whether on my device or theirs), it works
  • If I take over the dataset and refresh, I get the invalid credentials error
• I get the same issue regardless of browser or device being used

It's nothing to do with the Power BI file, Power BI Desktop version or options (e.g. privacy setting), browser, device, or a basic lack of SharePoint access - there's something specific to the way Power BI Service authenticates with SharePoint that is failing

Everything was working until one day last week, then suddenly it's stopped

I've contacted MS but historically have had more success with community support than MS support, so if anyone has any suggestions I'd love to know - has been two weeks so far struggling with this!