Hi Steffen,
Thank you for contacting Syncfusion support,
We have checked your reported issue, currently we (Syncfusion components) do not have support for .NET CORE 3.0 Preview 8. We are in process of migrating our components to Preview 8. It will be implemented in our upcoming NuGet release which is expected to be rolled on or before 21st August 2019.
Regards,
Saranya