Online operation cannot  be  performed  if  page  transfers  via
  memory are enabled.

  Explanation:  The backup, copy_database, or  move_area  facility
  is  not  able  to  handle modified database pages which have not
  been  written  to  disk.   This  situation  may  arise  if  page
  transfers via memory are enabled for global buffers.

  User Action:  Either disable page transfers via memory  for  the
  database or perform the operation off line.