I was able to resolve the problem by replacing the spaces in the UNC with "%20"
    'EMAIL BODY
        Dim MailTxt As String
            MailTxt = "<p>Attached is a copy of the updated Report.</p>" & _
                        "<a href=" & Replace(CStr(ActiveWorkbook.FullName), " ", "%20") & "> Report</a>"
I still don't understand why this stopped working after 2 years?