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Outlook for Mac 2016 Error code: -19874

Tim Cordell
Tim Cordell asked
Hello-   I am new to this group, but not new to Exchange (been working on it since v5.5).   I wanted to ask if anyone has seen this error message.  The Error is Could Not Synchronize record:  ##meeting name##: ##AccountName##

We are on office365 and i have a case opened already with MSFT on the issue, but wanted to see if anyone had already seen this issue.

The repro steps are:
1> have someone send you a meeting request.
2> Accept the meeting request.
3> The error should log within a minute under Sync Errors

Here's the error and help would be greatly appreciated :)
Error -19874
Watch Question

FYI : Am a Desktop tech that encountered this error for the first time today  ( 09/11/2017)

Error Code: "An internal server error occurred. The operation failed., An item with the same key has already been added."

Environment : Mac , Outlook 2016 ( 15.39 [ 171010] ) ,  Mac OS High Sierra 10.13.1 , Office 365.

Connectivity : Wi-Fi on client side with inline proxy , Status of connection on most days would be described as stressed ( Line utilization is very high in our org. ) [ Location South Africa ]

I tried clearing the cache of the calendar , same issue , from what I can see it's a database error.
I'm also seeing errors downloading attachments which usually happens as a result of "poor" connectivity on our end.

I'd like to see what results from the call you have with MSFT  .


Thank you for responding Mr. Mystikal, and I'm glad we aren't the only ones seeing this error.  I will let you know what and when i hear back from MSFT.
Hi There Tim,

I just received additional information from my affected user , at home he has a 100mb/ps Fiber line and the issue is present there as well. We are seeing a spike in issues where multiple users report attachments are greyed out , as per the below article MS is still working on it. Just wanted to check if you're seeing anything of that sort on your side , this coincidence shouldn't be overlooked.

Fixes or workarounds for recent issues in Outlook for Mac

We had these issues previously when latency to our 365 mail servers was very high [ over 15000 ms ] , the latency right now isn't fantastic , but nowhere near as tragic as previously experienced.

On Windows machines -> Outlook Connection Status.
Right now our Avg Resp ranges between 925 ms Foreground and 2313 ms Background.

Our tenant to my knowledge is in Ireland , no available tenants on African soil yet which would account for the latency.

Mr Mystikal
I also have a user experiencing the same issue. He has a Mac , Outlook 2016,  Mac OS High Sierra 10.13.1 , Office 365.

The user is in California. The error section says: Could not synchronize record: Call to Exchange server  user@domain.com

Details section says: An internal server error occurred. The operation failed., An item with the same key has already been added.

According to the user he has this issue with his calendar. He says every time he gets an invite or creates a meeting and after the event is finished or passes the error arrives.
Hey Tim,

we also have the same issue, but only the Mac devices are infected and I think after the last OS update started the problem.
Our Windows devices don't have this issue...

Do you have any information to fix the issue??

Best regards from Berlin



Hi everyone-  at this time I haven’t heard back from Microsoft on this issue, and there isn’t a fix or workaround that I know of.  It does sound like it just started happening.   I will reach out to Msft today to see what they say.
Just another update here......

On my MacBook Air running Sierra 10.12.6,  Outlook 15.39 (171010) using Office 365, I received the same error code: -19874

I accepted a meeting this morning and noticed the exclamation point in the lower right corner of the Outlook windows indicating there was a problem. Attached is the screen shot of the error messages I received.

I did log into https://portal.office.com to confirm if the event synced up there, and it did.

Hope this helps.
Just updated Outlook to Version 15.40 (171108)

Still same behavior of getting a meeting invite and upon accepting this I receive the synch error. I did 3 different tests on the same day (today) and each one received the synch error. Also tried removing the profile in Outlook and emptying the cache for the calendar. This did not help anything.

However, the event is still synching up to Office 365 cloud.


Thanks Tanner and everyone who has reported also seeing the issue. According to MSFT, there isn't a fix for this issue at this time, but They are looking into it. Will keep this thread alive if I hear back something.

Here is what I am running and also receiving the same error message, so it sounds like it isn't an outlook specific version issue.  

Outlook for Mac 2016 Diagnostic Log
Outlook for Mac 2016 Build: 16.8.17110100
Activation Type: Subscription
Microsoft AutoUpdate Build: 3.11.17101000
Mac OS Version: Version 10.13 (Build 17A405)
Audience: Insiders_DevMain
AudienceGroup: Insiders
Channel: DevMain
Computer name: [Not Logged]
Current Time Zone: America/Los_Angeles
Default Profile Name: [Not Logged]
Profile Path: [Not Logged]
Proxy Settings:
[Not Logged]


Hi again-  So I found out today that there is a fix for this, but it will happen on the Exchange side of the Office365 service.   So we just wait, and ignore the errors.   I was told the fix will be rolled out "within a few weeks" globally.    Stay tuned...
Thank you Tanner and Tim , you've saved me a few hours of wasteful troubleshooting , was about to try force a DB rebuild client side. Have a great "few weeks" :)
No problem Mr. Mystikal!  Hope this fix is rolled out sooner than later. Glad to hear that its not on our end. Tim thank you for your help as well. Please keep us updated if you hear of the fix.
Can i have the link to the Thread that has the issue you mentioned.
I would need it to show my client the solution is on its way.
Hello Tim,

Thank you for the latest information about this error.
Is there a link to track about this fix? (or any sort of information)
Thanks for the info @Tim. I will let my affected users know as well. I second the request for a link or thread that I can note in my ticket that it's being worked on.


hi everyone. Unfortunately, i haven't seen anything public, but as an office365 customer, you are able to open a case under Service-Support.  Since this is a bug with the service, it's free of charge.  Here is the link: https://portal.office.com/support/support.aspx
Hi again everyone.  Great News!   I received word this morning that the fix has been rolled out.  I can confirm that we are no longer able to repro the error.
I tested this on my laptop and I have not received any synch errors for the last 2 days.

I also checked in with my user who first reported the issue. He also confirms that he does not receive any synch errors with his calendar.
Confirmed it appears issue resolved , thanks everyone. Think it's safe to close off this thread.
Same here, confirmed that MAC users are no longer receiving that error message.
Issue was resolved by bug-fix on Office 365 Exchange servers as highlighted and tracked by Tim.