The TypoScript setup variable plugin.tx_seminars_pi1.what_to_display contains the value "seminar_registration", but only the following values are allowed:
(seminar_list, single_view, topic_list, my_events, my_vip_events, list_registrations, list_vip_registrations, category_list)
This value specifies the type of seminar manager plug-in to display. If this value is not set correctly, the wrong type of plug-in will be displayed. Please fix the TypoScript setup variable plugin.tx_seminars_pi1.what_to_display in the plugin Flexforms or in your TypoScript template.
The configuration check for this extension can be disabled in the extension settings in the backend: Admin Tools > Settings > Extension Configuration