The access to the connected private and/or public Outlook contact folders, which were integrated via the XPhone Connect Directory as data source, is no longer possible after some time. In the Windows Event Viewer there is a MAPI error 8004011D or "MAPI_E_FAILONEPROVIDER" in the application log.
The XPhone Connect Directory opens up to 25 sessions for the data source "Private Outlook Contact Folder" (as required) and another 5 sessions per data source "Public Outlook Contact Folder" in order to be able to perform search queries promptly. Thus it can happen that more than 32 sessions are opened for the MAPI user under which the XPhone Server service account runs. This is not possible with the Exchange Server's default settings.
You can increase this limitation of the Exchange Server by means of a registry entry, afterwards one of the Exchange services has to be restarted. Proceed as follows on the Exchange Server:
Registry entry (here e.g. increase to 128 sessions):
"Maximum Allowed Sessions Per User"=dword:00000080
The "Microsoft Exchange Information Store" service must then be restarted. The mailboxes are not accessible during the restart!
There is also a so-called NSPI session limit, which Microsoft has introduced with Windows 2008 Server-based domain controllers (see http://support.microsoft.com/kb/949469/en-us ). More than 50 NSPI sessions per user and domain controller are not allowed. This limit can also be raised via the registry on all relevant domain controllers (this must be done on all domain controllers):
"NSPI max sessions per user"=dword:00000200
If this does not solve the problem on the XPhone Connect Directory Server with the Outlook data sources immediately, the domain controller must be restarted.