Hybrid Calendar working for some, not others

[aph3i]

Cisco Support has been giving me the round about, so posting on the forum to see if someone has seen this before or not. We have 700 users who's Hybrid Calendar is working, while 1200 users are not. The error we see on the Webex admin portal is the following:

Received "notSpecified: The request failed. microsoft.exchange.webservices.data.ServiceResponseException: The account does not have permission to impersonate the requested user." from Exchange Server: "<address>".

I've worked with Microsoft Exchange team to configure the impersonation account making sure a few more permission steps were granted outside of just the documentation provided from Microsoft. Seeing as 700 users work, while 1200 users don't, I really don't think this is a Microsoft Exchange issue, and more a Webex issue.

Anyone have any ideas?

[ChandrakanthShenoy2016]

Is this 0365 or Exchange on-prem. Assuming this is on-prem, you may want to check with the Exchange team on the Throttling policy as documented here on Cisco site. This may be a good point to start the troubleshooting.

[Jonathan Schulenberg]

Conversely, if this is O365 be aware that there is a cap of 1000 users-per Expressway cluster and related O365 service account.
https://collaborationhelp.cisco.com/article/en-us/nv5p67g#reference_7F131AABFF2822A26267C3255629F5F7

A ince the total users easily exceed that cap it’s possible that throttling on that service account is dragging this down. A permissions issue seems more likely though considering the error. I suggest following the guide EXACTLY - nothing extra - and ensuring this service account isn’t being shared by anything else.