Every folder shows '?', meaning 'The item can't be found'
A new development, I think, or else I did not notice it before: every folder, when opened, shows a large '?' at the top left corner. If I click on it, a message appears, saying 'The item can't be found'. But the items contained in that folder are already there, clearly accessible,in that very folder! What is all this about? Can I get rid of the ominous question-mark globally'?
Mac mini, OS X Mountain Lion (10.8.5)