Calendars no longer sync across devices after iOS/Mac OS X updates

Question

Ever since updating my MacBook Air to Yosemite and my iPhone 6 to iOS 8.0.2 (or maybe 8.1.1), my calendars no longer sync correctly across devices. Prior to the updates, any addition or change I made to the calendar on my iPhone would be added to the calendar on my Mac the next time I synced my phone. Now I have duplicate events on my phone, the calendar on my Mac no longer incorporates changes from the phone and I get a dialog box on the Mac 2-3 times an hour about calendar conflicts. No matter how many times I review the conflicts and choose which events/updates to save, the box keeps coming back. I've never had a persistent issue like this with any previous iOS or Mac OS updates and I can't figure out why it won't stop. Help, please!

Answer 1

OK, that was easy enough. Here's what I took from the log when I synced a few minutes ago. I kept the log entry for one of the multiple calendar sync conflicts.
12/7/14 2:30:15.753 PM    iTunes[13010]    Entered:_AMMuxedVersion2DeviceConnected, mux-device:1
12/7/14 2:30:15.761 PM    iTunes[13010]    tid:15143 - unable to query device capabilities
12/7/14 2:36:19.054 PM    CalendarAgent[269]    [com.apple.calendar.store.log.caldav.queue] [Account refresh failed with error: Error Domain=CoreDAVHTTPStatusErrorDomain Code=401 "The operation couldn’t be completed. (CoreDAVHTTPStatusErrorDomain error 401.)" UserInfo=0x7fc656382120 {AccountName=xxxx xxxxxxxxxx, CalDAVErrFromRefresh=YES, CoreDAVHTTPHeaders=<CFBasicHash 0x7fc655f0b9c0 [0x7fff72fc9cf0]>{type = immutable dict, count = 9,
entries =>
    0 : x-fb-rev = 1522031
    1 : Content-Type = <CFString 0x7fc65633a370 [0x7fff72fc9cf0]>{contents = "text/plain; ; charset="utf-8""}
    2 : Pragma = no-cache
    6 : Date = <CFString 0x7fc6537cf400 [0x7fff72fc9cf0]>{contents = "Sun, 07 Dec 2014 20:36:18 GMT"}
    8 : x-fb-debug = <CFString 0x7fc656337440 [0x7fff72fc9cf0]>{contents = "H4G9L5Ap4OdEAtTQrPmOWpVTPSxhVBRB0vBO8rDrT77EF6RpVc/hNfpRKJkvRfWnWsQvWyWWfh6urs K4jjfLNQ=="}
    9 : Www-Authenticate = <CFString 0x7fc655f3cc70 [0x7fff72fc9cf0]>{contents = "OAuth realm="Facebook WebDAV", Basic realm="Facebook WebDAV""}
    10 : Content-Length = 40
    11 : Expires = <CFString 0x7fc6563410a0 [0x7fff72fc9cf0]>{contents = "Sat, 01 Jan 2000 00:00:00 GMT"}
    12 : Cache-Control = <CFString 0x7fc6537c9a80 [0x7fff72fc9cf0]>{contents = "private, no-cache, no-store, must-revalidate"}
12/7/14 2:36:19.206 PM    CalendarAgent[269]    [com.apple.calendar.store.log.caldav.queue] [Coalescing failed operation [<CalDAVAccountRefreshQueueableOperation: 0x7fc6534d5ed0; Sequence: 0>]]
12/7/14 2:36:26.777 PM    discoveryd[48]    Basic Sockets SetDelegatePID() failed for PID[5784] errno[3] result[-1]
12/7/14 2:36:45.639 PM    discoveryd[48]    Basic Sockets SetDelegatePID() failed for PID[5784] errno[3] result[-1]
12/7/14 2:36:57.738 PM    iCalExternalSync[27294]    Core Data: warning: validation recovery attempt FAILED with Error Domain=NSCocoaErrorDomain Code=1560 "Multiple validation errors occurred." UserInfo={
    CalCalendarUID = "41EAC630-54F8-42A4-A528-B38FF114C8BD";
    CalManagedObjectType = CalManagedEvent;
    NSDetailedErrors =     (
        "Error Domain=NSCocoaErrorDomain Code=1570 \"localUID is a required value.\" UserInfo=0x7ffbde190ab0 {NSValidationErrorKey=localUID, NSLocalizedDescription=localUID is a required value., NSValidationErrorObject=<CalManagedEvent: 0x7ffbdb5a5e20> (entity: Event; id: 0x7ffbdb51ba70 <x-coredata://6119C894-3173-4694-918E-26A2EA230F14/Event/p518> ; data: {\n    amIInvited = 0;\n    attachments =     (\n    );\n    attendeeComments =     (\n    );\n    attendeeIconState = 0;\n    attendees =     (\n    );\n    birthdayName = nil;\n    cachedLocalDefaultAlarmDeleted = 0;\n    cachedServerDefaultAlarmDeleted = 0;\n    calendar = \"0x7ffbdb50c140 <x-coredata://6119C894-3173-4694-918E-26A2EA230F14/LocalCalendar/p5>\";\n    calendarServerAccess = nil;\n    changeKey = nil;\n    classification = 0;\n    creationDate = \"2014-10-17 00:46:55 +0000\";\n    customAlarms =     (\n    );\n    dateStamp = \"2013-01-28 01:44:49 +0000\";\n    deleteSyncRecord = nil;\n    dontNotify = nil;\n    dontSchedule = 0;\n    dropBoxFreeBytes = 0;\n    dropBoxLocation = nil;\n    dropBoxLocationOverrideBaseURL = nil;\n    dropBoxLocationOverrideBaseURLString = nil;\n    eTag = nil;\n    endDate = \"2012-12-12 00:00:00 +0000\";\n    endTimeZone = nil;\n    hasAlarm = 1;\n    hasAttachment = 0;\n    hasAttendee = 0;\n    hasDropBox = 0;\n    hasRecurrenceException = 0;\n    invitationMode = 0;\n    isAllDay = 1;\n    isBirthdayWithoutYear = nil;\n    isCalendarOwnerInvited = 0;\n    isDetached = 1;\n    isInvitationDirty = 0;\n    isInvitationSent = 0;\n    isMyResponseDirty = 0;\n    isOrganizerMe = 1;\n    isPhantom = 0;\n    isRSVPNeeded = 0;\n    isVeryLongDuration = 0;\n    isYearlessLeapMonthBirthday = 0;\n    itemID = nil;\n    linkID = nil;\n    localDefaultAlarms =     (\n        \"0x7ffbde649710 <x-coredata://6119C894-3173-4694-918E-26A2EA230F14/Alarm/p785>\"\n    );\n    localUID = nil;\n    location = nil;\n    lunarCalendarID = nil;\n    masterException = \"0x7ffbdb5a27e0 <x-coredata://6119C894-3173-4694-918E-26A2EA230F14/RecurrenceException/p77>\";\ n    messages =     (\n    );\n    myAttendee = nil;\n    myAttendeeStatus = ACCEPTED;\n    needsReply = nil;\n    needsServerConfirmation = 0;\n    notes = \"Paid 12/29\";\n    omitSyncRecord = 0;\n    organizerAddress = nil;\n    organizerAddressString = nil;\n    organizerCommonName = nil;\n    organizerEmailAddress = nil;\n    organizerScheduleAgent = nil;\n    organizerScheduleStatus = nil;\n    permission = 0;\n    previousProdID = nil;\n    privateComment = nil;\n    recurrenceEndDate = nil;\n    recurrenceExceptions =     (\n    );\n    recurrenceRule = nil;\n    scheduleTag = nil;\n    sequence = 0;\n    serverDefaultAlarms =     (\n    );\n    serverFilename = nil;\n    sharedUID = nil;\n    sliceChild = nil;\n    sliceParent = nil;\n    startDate = \"2012-12-11 00:00:00 +0000\";\n    startDateIncludingTravel = \"2012-12-11 00:00:00 +0000\";\n    status = nil;\n    structuredLocation = nil;\n    syncMalfunctionDetected = nil;\n    syncNeeded = 0;\n    timeZone = nil;\n    title = \"Express bill\";\n    transparency = 0;\n    travelStartLocation = nil;\n    unlistedOtherAttendees = 0;\n    unrecognizedICSProperties = \"0x7ffbdb5a2720 <x-coredata://6119C894-3173-4694-918E-26A2EA230F14/ICSElementProperties/p757>\" ;\n    url = nil;\n    urlString = nil;\n})}",
12/7/14 2:36:57.821 PM    iCalExternalSync[27294]    [ICalExternalSync ]Failed Event/p517 modify validation - Multiple validation errors occurred.(localUID is a required value.; sharedUID is a required value.)
12/7/14 2:36:57.830 PM    iCalExternalSync[27294]    [ICalExternalSync ]Failed Event/p518 modify validation - Multiple validation errors occurred.(localUID is a required value.; sharedUID is a required value.)
12/7/14 2:36:57.835 PM    iCalExternalSync[27294]    [ICalExternalSync ]Error: Core Data save failed. Sync canceled. - Multiple validation errors occurred.(localUID is a required value.; sharedUID is a required value.)
12/7/14 2:36:57.892 PM    iCalExternalSync[27294]    [ICalExternalSync ]Multiple validation errors occurred.
12/7/14 2:37:06.791 PM    AppleMobileSync[27302]    [0x10031d6d0] |ISyncManager|Warning| Could not retrieve ISyncClient with id com.apple.Keychain!