one client (it's allways the same client) has often problems with mapi sessions killed by the exchange server. The Application Eventlog on the exchange logs eventid 9646 with source MSExchangeIS:

Die MAPI-Sitzung '/o=xx/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=xxxx' hat die maximal zulässige Anzahl von 250 Objekten vom Typ 'objtMessage' überschritten.

The client has no eventlogs logged about this error.

I looked for installed Outlook Add-Ins and found the default add-ins from microsoft, an adobe pdf add-in (which I deactivated because it's not needed) and an "Octopus" plugin from telekom.

Octopus is a CTI-application that connects to Outlook. My guess is, that Octopus (or its add-in) causes this error because this client has over 1100 contacts.

My question is: how can I find out, which application/add-in causes this problem?

Edit: I already looked at eventid.net but nothing helped.

Edit2: Exchange-Cache-Mode is not used nor are there any shared folders / mailboxes open.