Mail Backup under Windows 8.1

I have been successfully backing up Windows Mail for over a year (moved from Vista - not the Windows 8 one) and have recovered from a backup successfully too.

Since updating Windows 8 to Windows 8.1, the backup paused with the error "Failed to backup file or folder 'windows-mail:/' Retry Ignore Ignore All Cancel".

Choosing Ignore, the backup then appears to proceed and validate as normal, but I have no idea whether the backup is actually working. I suspect not because the progress bar fills very quickly.

Does anyone have an explanation or solution?