I updated Piwik from 2.14.3 to 2.16.0 and my log files were filled with this notice :
PHP Notice: Undefined index: create_new_visit_after_midnight in /usr/share/piwik/plugins/CoreHome/Tracker/VisitRequestProcessor.php on line 158
When I say "filled", I say 40K notices per hour (my install is fairly large).
I had to downgrade to 2.14.3.
global.ini.php wasn't updated correctly. There should be a config value set for
create_new_visit_after_midnight within the
Tracker section after update. If not maybe you need to reupload the