When i upgraded to the latest eM Client V10.4.5326 Mac OS 26.4.1 & iOS V10.4.5267 OS 26.4.1 on iPhone and iPad, all my iCloud account Calendars and Contacts disappeared in eM Client. The accounts were not even showing at all.
So i removed and re-added the iCloud accounts on all devices via the automatic account wizard again and the Calendars and Contacts then all re-appeared ok.
So just incase anyone else finds they disappeared when upgrading to the latest apps.