I tried manual backup command as well but ended up getting same error message. So I can conclude that its not related to batch file as manual tableau admin backup command also giving same error message.
how large does your backup typically end up? Also, what release are you on? My going in hunch is that one of the tde files within the dataengine directory is somehow locked by another process and somehow cannot be incorporated into the backup. Is it possible to do a tabadmin restart and then try it again?