Reestablishing the Personal Vault allows the synch to proceed. It was the failed synching that caused the Red X.

In some cases there will be old 'uncached' files that appears. Open it (you will have to authenticate to open).Find your Personal Vault in your file/folder list.Click on OneDrive in the upper left corner of the browser screen.A pop-up stating, 'Your Personal Vault is back'.If you closed the window, select Settings in the upper right of online OneDrive (to the left of the question mark), select options, select Personal Vault from the left side menu, then select Enable.Go back to the browser and select Enable on the Persona Vault window if it is still opened.Wait 30-60 seconds for your OneDrive desktop to synch.Select the options to Disable the Personal Vault.A message 'Cant delete Personal Vault' appears. Select the Personal Vault and attempt to delete it.Disable the Personal Vault online via the Web browser.Verify all your important files have been successfully moved by verifying the folder and file count.Refresh and verify the Personal Vault is empty.

Move all the files and folders out of the Personal Vault to another folder in your OneDrive.Count all the files and folders presently in the Personal Vault.Open the vault via the Web Browser and select all the files and folders in the vault.Access the personal vault OnLine via the the View OnLine option from the desktop icon in the task bar.I was able to get rid of the Red X on my OneDrive icon and the error message saying that the Personal Vault could not be synchronized by doing the following: OneDrive Personal Vault cannot be unlocked on Windows10.

Microsoft personal vault