My attempt to upgrade from 2.15 to 2.16 through browser failed after some time with this error message:
Fatal error: Class ‘ComposerAutoloaderInitdXXXXXXX’ not found in /XXX/piwik/vendor/autoload.php on line 7
The upgrade has made enough changes to the files to ruin the whole piwik install. Any call to piwik/index.php returns the same error.
Any clue on what to do before attempting a fresh re-install from scratch?