Thanks !
Here the content of nextcloud.log file after re-launching the upgrade command with “-vvvv” option:
{"reqId":"ppZfrh7h8x6y\/OLEKO52","remoteAddr":"","app":"cli","message":"Memcache \\OC\\Memcache\\Redis not available for local cache","level":1,"time":"2016-10-30T13:10:26+00:00","method":"--","url":"--","user":"--"} {"reqId":"ppZfrh7h8x6y\/OLEKO52","remoteAddr":"","app":"cli","message":"Memcache \\OC\\Memcache\\Redis not available for distributed cache","level":1,"time":"2016-10-30T13:10:26+00:00","method":"--","url":"--","user":"--"} {"reqId":"ppZfrh7h8x6y\/OLEKO52","remoteAddr":"","app":"core","message":"starting upgrade from 9.0.53.0 to 9.1.1.5","level":0,"time":"2016-10-30T13:10:29+00:00","method":"--","url":"--","user":"--"} {"reqId":"D614z\/oQWHo9NXQ8uggi","remoteAddr":"","app":"cron","message":"Update required, skipping cron","level":0,"time":"2016-10-30T13:15:05+00:00","method":"--","url":"--","user":"--"} {"reqId":"ppZfrh7h8x6y\/OLEKO52","remoteAddr":"","app":"core","message":"Exception: {\"Exception\":\"Exception\",\"Message\":\"files can't be disabled.\",\"Code\":0,\"Trace\":\"#0 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/legacy\\\/app.php(423): OC\\\\App\\\\AppManager->disableApp('files')\\n#1 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/Updater.php(433): OC_App::disable('files')\\n#2 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/Updater.php(277): OC\\\\Updater->checkAppsRequirements()\\n#3 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/Updater.php(151): OC\\\\Updater->doUpgrade('9.1.1.5', '9.0.53.0')\\n#4 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/core\\\/Command\\\/Upgrade.php(292): OC\\\\Updater->upgrade()\\n#5 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/3rdparty\\\/symfony\\\/console\\\/Command\\\/Command.php(259): OC\\\\Core\\\\Command\\\\Upgrade->execute(Object(Symfony\\\\Component\\\\Console\\\\Input\\\\ArgvInput), Object(Symfony\\\\Component\\\\Console\\\\Output\\\\ConsoleOutput))\\n#6 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/3rdparty\\\/symfony\\\/console\\\/Application.php(844): Symfony\\\\Component\\\\Console\\\\Command\\\\Command->run(Object(Symfony\\\\Component\\\\Console\\\\Input\\\\ArgvInput), Object(Symfony\\\\Component\\\\Console\\\\Output\\\\ConsoleOutput))\\n#7 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/3rdparty\\\/symfony\\\/console\\\/Application.php(192): Symfony\\\\Component\\\\Console\\\\Application->doRunCommand(Object(OC\\\\Core\\\\Command\\\\Upgrade), Object(Symfony\\\\Component\\\\Console\\\\Input\\\\ArgvInput), Object(Symfony\\\\Component\\\\Console\\\\Output\\\\ConsoleOutput))\\n#8 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/3rdparty\\\/symfony\\\/console\\\/Application.php(123): Symfony\\\\Component\\\\Console\\\\Application->doRun(Object(Symfony\\\\Component\\\\Console\\\\Input\\\\ArgvInput), Object(Symfony\\\\Component\\\\Console\\\\Output\\\\ConsoleOutput))\\n#9 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/Console\\\/Application.php(146): Symfony\\\\Component\\\\Console\\\\Application->run(Object(Symfony\\\\Component\\\\Console\\\\Input\\\\ArgvInput), Object(Symfony\\\\Component\\\\Console\\\\Output\\\\ConsoleOutput))\\n#10 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/console.php(90): OC\\\\Console\\\\Application->run()\\n#11 \\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/occ(11): require_once('\\\/var\\\/www\\\/html\\\/s...')\\n#12 {main}\",\"File\":\"\\\/var\\\/www\\\/html\\\/site\\\/nextcloud\\\/lib\\\/private\\\/App\\\/AppManager.php\",\"Line\":258}","level":3,"time":"2016-10-30T13:16:56+00:00","method":"--","url":"--","user":"--"}