@jeremiah808 You can use a calendar app that can export and import events, or export the events and then import then in the Disroot Web interface. aCalendar+ or Business calendar should be able to do that. Maybe there are also apps that can copy calendar entries from one calendar (non-DAVx5) to another (DAVx5).
Ok, found something myself here: https://github.com/owncloud/apps/issues/687. Seems like Owncloud is not really good at dealing with some kind of either corrupted, or just “different” .ics imports. So on the owncloud user page I downloaded the not working calendar as .ics file, deleted the owncloud calendar, created a new calendar with the same name, imported the ics file. Seems like everything works now.