I've been experiencing loads of issues with calendars recently.

Creating a new appointment causes other appointments to duplicate over and over again! I've recorded a screen capture of this happening and sent it over to ST (ticket 023-247AC283-0010).

I've also noticed body text from completely different appointments appear for some of these new duplicate appointments.

Constantly getting sync errors with Outlook 365 too.

To confirm, I use Outlook 365 on Windows 10 (IMAP for emails and SharePoint for calendars - I have two calendars set up) and Outlook for iOS with EAS on my iPhone X (lastest iOS).

I've noticed this as well.  I'm not seeing anything in the ActiveSync logs, but don't really know what to look for.

Besides a new <ServerId>, the TimeZone looks wierd:

[2019.06.12] 08:52:22.616 [1321703609]             <TimeZone xmlns="Calendar">AAAAAFUAVABDAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFUAVABDAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==</TimeZone>

It seems ilke it only occurs on events added through SharePoint.  Same events added in CalDAV or ActiveSync are not duplicated.

we are also on version 7090 after upgrading from V15 having issues with calendars failing to add to outlook after the upgrade to 7090.

I have a client that has 8 shared calendars and a front office secretary that adds appointments to all calendars.  every computer has these calendars added from webmail to outlook and we are getting send receive errors after the upgrade to 7090.   all computers have outlook 2013 or greater.

I picked one account to start troubleshooting this and I cleared all calendars from outlook, cleared all connections from webmail and started fresh.  we login as the user and see all the shared calendars, going one by one we add them to outlook and two added fine then when we get to the third where we start getting send receive errors.

Along with all of this we have a huge amount of duplicates being created.

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Here is some addition info for clarification:

8 calendars are shared to everybody with FULL permissions.

1)STEPS FOR FAILURE:
I login as a regular user > go to calendar > select ADD TO OUTLOOK > pick a calendar that is shared to this user full permission (they all do anyway) > add it to outlook and watch the send receive fail.  ERROR MESSAGE is calendar not found.

2)STEPS THAT WORK:
I get the password for the person I need to see the calendar and login as them > go to calendar > add to outlook and it will work fine.

now the idea would be for the permissions to follow down to outlook and I would assume I should be able to add the calendar from a user that has it shared to as full permissions.

I have a ticket open on this and hope for resolution soon.

I've had an update to my ticket (023-247AC283-0010) stating that the devlopers are aware of this and will be releasing a fix.

To confirm, my issues have been duplicate appointments and incorrect body text (being used for these duplicate appointments).

I also had an update to my ticket, but no time frame on resolution. to add:  I am getting random duplicates on server reboot and also duplicates in the below scenario. 

scenario to add duplicate:

all outlook computers have calendars added by logging into the customer account and adding one by one not using the shared calendar in eas which fails.

outlook computer 1 running 2016 adds an appointment and it appears everywhere (webmail, outlook, phone EAS).

outlook computer then 2 updates that same appointment with a different title.  that update makes it to the phone and webmail, but not to any other add to outlook computer.

if I go to webmail and update the same appointment changing the title slightly then all outlook computers show the original appointment and one duplicate appointment with the new title.