OneDrive for Business states that “Unsupported server. The server you are trying to access is using an authentication protocol not support by this version of Office.”
(h/t Microsoft forum)
- End the GROOVE and MSOSYNC processes.
- Open Word / Excel / PowerPoint (or any Office app that is not OneDrive For Business).
- Click the Switch Account at the top right.
- Click Add Account.
- Add the account that you actually need to sync that particular library.
- Close the application.
- Close OneDrive for Business. (You need to actually right click in the taskbar and click Exit).
- Open OneDrive, and attempt to sync the library. You should not be prompted with the error message.
* If these steps do not work, please remove all signed in accounts, and then attempt to sync the library which should prompt appropriately. After the library syncs, feel free to add those identities back