What should I do when macOS can no longer open its Help Viewer?

The Help Viewer used to display the online documentation of applications suffers from several technical defects, so it might stop working correctly from time to time. To repair it, perform the following steps:

  1. Open the pane User.
  2. Select the tab item Repair.
  3. Press the button Repair now in the section Repair “Help Viewer”.

Further information: The Pane User.