No caching plugin. Turns out when I access the settings page directly it gives the wp_die response. After going to the settings page through the link in the left menu it gives the correct settings page. So it is (sort of) solved. Fyi the difference in the links: This works >Settings page left menu: ‘…wp-admin/options-general.php?page=wp-matomo-settings’. This has no access > Directly from plugin menu: ‘…wp-admin/options-general.php?page=wp-piwik/classes/WP_Piwik.php’.
Minor other thing after installing 1.1.2 it shows this version in the text: “WP-Matomo 1.0.31 is successfully connected to…”. This old version number seems hardcoded in the plugin repository: classes/WP_Piwik.php.
Thanks for your quick response!