After update PIWIK to 3.0.4 from 2.16… There are a warining displayed on the top of almost all reports:
WARNING: /var/www/html/piwik/plugins/CustomDimensions/Reports/GetCustomDimension.php(285): Notice - Undefined index: idSites - Piwik 3.0.4 - Please report this message in the Piwik forums: http://forum.piwik.org (please do a search first as it might have been reported already)
You can remove any plugin from PIWIK_HOME/plugins/ directory and reinstall it from Piwik distr or from piwik marketplase. “Reinstall” mean just copy the plugin from distr or marketplace to the plugins folder and then activate the plugin. You can activate the plugin using piwik console “piwik plugin:activate”, or using “plugins” menue in your admin web-interface.