When apps such as e.g. https://runningcoach.me/ create trainings for today, they are synced correctly to the app. If one or more of those trainings are later removed, they disappear from https://connect.garmin.com/modern/calendar but are still visible in the app. If clicked on it, an error message is shown similar to "Content could not be loaded. Please try again later." (German: Inhalt konnte nicht geladen werden. Bitte später erneut versuchen.)
Restarting the app does not help (normally this helps with e.g. missing sleep state).