Hi Gert,
The DayPilotModule class is decorated with @NgModule (the decorators are not visible in the daypilot-angular.min.d.ts file - they get compiled to daypilot-angular.min.metadata.json).
DayPilot should work fine with the Ivy compiler. You can try creating a new Angular 8 project using the UI Builder (https://builder.daypilot.org) and add the "enableIvy" option to tsconfig.app.json.
This error message is a bit misleading and it sometimes appear if you upgrade an existing project to a higher Angular version.
I recommend deleting the node_modules folder and running "npm install" to reload the dependencies. If that doesn't help, it might be necessary to create a new Angular 8 project (using "ng new") and copy the source files over to this new project.
Please let me know if it doesn't help.