<str> is not a valid backup file

  Explanation:  The backup file specified could not be used.   The
  specification  may  be  in error, or there may be an operational
  error such as an improperly  mounted  tape  or  a  file  damaged
  during  transmission.   If  you  have  issued  the  RMU  Restore
  command, the file format may not be valid.

  User  Action:   Correct  the  backup   file   specification   or
  operational error.  In the case of a disk-format backup file, if
  the OpenVMS file attributes have been damaged or lost, it may be
  possible  to  reset  them  with  a  command  similar to SET FILE
  /ATTRIBUTE=(MRS:32256,LRL:32256,RFM:FIX).