For the last several days, I have had this message in Word/Excel/PowerPoint when I have been editing files that are synced to my machine using OneDrive for Business.
It also appears when I open a file direct from Word Online, SharePoint Online or Teams.
I have tried stopping the OneDrive sync, removing the files and then re-syncing, but I still get the same message.
My files that are stored in OneDrive for Business and not in SharePoint Online are fine and can be edited and synced as expected.
I am using Windows 10 Pro Build 1803
As of 12 June, following Windows updates, this issue has disappeared