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OCS 2007 R2 IM Conferencing failed with 603 decline error.

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If you have 3rd certificate assigned in the FE server, you may failed to do IM conference and below error may occurs:

603  Decline

ms-diagnostics:  3098;reason="No MCU Factory Available";source="FE.contoso.com"

The resolution is:

Create DWORD registry under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\

Call it “SendTrustedIssuerList” with value of 0, default value. 

The reason is that since the IM conferencing server using the certificate from third party vendor, the MCU component may not display the third party certificate to itself (loop back) since this certificate is not in trusted certificate authorities list. That’s why we need set the SendTrustedIssuerList key to false.