Configuration check warning:
The TypoScript setup variable plugin.tx_seminars.attendancesPID is empty, but needs to be non-empty. This value specifies the page on which registrations will be stored. If this value is not set correctly, registration records will be dumped in the TYPO3 root page. If you explicitely do not wish to use the online registration feature, you can disable these checks by setting plugin.tx_seminars.enableRegistration and plugin.tx_seminars_pi1.enableRegistration to 0. Please fix the TypoScript setup variable plugin.tx_seminars.attendancesPID in your TypoScript template setup.
When that is done, please empty the FE cache and reload this page.
The configuration check for this extension can be disabled in the extension manager.