Calendar keeps updating/syncing for hours

Sometimes when I wake up my Mac in the morning, my Calendar App starts to update/sync with iCloud (looking for changes myde on my other devices or new invitations) – and then remains in that mode for hours. I can´t get it to really sync and show new events by re-opening the app or pressing comand-R. Often a restart of the Mac does´nt solve that. I thought it might be a problem with iCloud service not being available but if I browse to iCloud.com I can reach the service and see the new entries in the calendar. Any suggestion?

Till