I have been importing a lot of data using feeds, then deleting all imported items again when there were problems with the import. Unfortunately, the URL aliases for the pages do not seem to be deleted, so they are related to nodes that no longer exist. Is there any way to remove them, other than going through by hand?
A core bug causing this error was fixed for Drupal 8.2 and 8.3:
The fix for this problem, then, should simply be updating Drupal (...after having waited several months for this fix to be made and 8.2 released— but a satisfying answer now, i hope!)