Hey Ruud,
Thanks for reaching out to us!
Unfortunately we’re unable to provide support for the free version of our plugin here in the Pre-Sales forum. If you would like, you can reach out for more community support within our WordPress.org forum.
As a start for further testing, try following the Testing for Conflicts procedures prior to attempting an import via CSV. There could potentially be a conflict with another plugin or your existing theme that is causing the events to not import as expected.
Thanks!