You can't view free/busy information on another user's Calendar in Exchange Online
Original KB number: 2962513
You cannot view free/busy information for a specific user in Exchange Online. You experience this issue in Outlook Web App and Microsoft Outlook (including Outlook for Mac 2011). Specifically, you experience the following symptoms:
When you try to view the user's Calendar in Outlook, you receive the following error message:
could not be updated
When you try to view the user's Calendar through Outlook Web App, you see a red triangle with an exclamation mark.
This issue occurs if the user's Calendar has a time slot that contains more than 1,000 entries.
Delete unwanted items from the user's Calendar so that there are fewer than 1,000 items in that particular time slot.
To confirm that this is the issue that you're experiencing (if the user is running Outlook 2010 or Outlook 2007), follow these steps:
Enable troubleshooting logging in Outlook. For more info about how to do this, see How to enable global and advanced logging for Microsoft Outlook.
%temp%\OLkASfolder, look for the availability service log file that's named <date>-<time>-AS.log or <date>-<time>-fb.log.
Search the log file for an entry that resembles the following:
<MessageResponseClass="Error"><MessageText>Microsoft.Exchange.InfoWorker.Common.Availability. ResultSetTooBigException: The result set contains too many calendar entries. The allowed size = 1000; the actual size = <xxxx>
The number that's represented by the <xxxx> placeholder. If you're experiencing this issue, this number will be more than 1,000.
Still need help? Go to Microsoft Community.