According to ME2469722
, this can occur due to mobile device connections and/or issues with the device software. These problems may be the result of resource depletion on the Exchange server. See the article for more details.
According to some support forums, many instances of this event are caused by the third party software or some network devices affecting the communication between front-end and back-end server.
From a support forum:
We initially deployed a few Apple devices about 6 months ago - iPhones and iPads - and all had been working fine until recently, when this problem surfaced. Following is the response from Microsoft:
"First, it appears the customer is hitting an issue that is caused by Apple’s ActiveSync implementation; we have two bugs opened with Apple at this time for this issue. The Apple device is attempting to sync a large amount of data (typically, this is the contacts folder) but has set a “timeout” period for syncing the data of x minutes (I believe this is 4 minutes, but I may be mistaken). When the sync has not completed in the time allotted, Apple launches another thread re-requesting the full sync. When that thread cannot sync in it’s time allotted, the Apple device launches another thread…and so on. This continues until we run out of resources to satisfy further requests/connections. The user performing the iPhone sync in these dumps is firstname.lastname@example.org. The only workaround, at this time, is to deny the sync request (turn off syncing for the user account); we are awaiting a fix from Apple."
This indicates that some process is affecting the communication betweent he ActiveSync component of Exchange and the the Exchange mailboxes. Some support forum mention that in some cases, a server-level antivirus application may be the culprit.
provides generic information on troubleshooting ActiveSync while ME905013
describes various timeout intervals that are configured for the various components involved with ActiveSync.