MSExchangeIS 9646 client type MoMT exceeded the maximum of 32 objects of type Session

Hello, we're running Exchange 2013 CU3. When a user tries to open multiple shared calendars in Outlook 2007 she gets disconnected from the server. On the server the event 9646 is logged. Mapi session /o=XX /ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=x.y with client type MoMT exceeded the maximum of 32 objects of type Session. Tried to use ThrottlingPolicies and

HKLM\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem\MaxObjsPerMapiSession

but nothing helped. Seems that Exchange 2013 is different.

Has anyone a solution for this problem?

Thanks in advance

Mike