A quick update on the reported issues with installing OCS 2007 R2 on machines that have .NET 4.0.
In a nutshell, if you try to install any of the OCS 2007 R2 roles or Group Chat Server, on a server that has the .NET 4.0 Framework installed along side the .NET Framework 2.0 or .NET Framework 3.5 Service Pack 1 (SP1), you’ll get an error.
A Knowledge Base article was released this morning that provides more details and gives a workaround: You cannot install Office Communications Server 2007 R2, Unified Communications Managed API 2.0 Core Redist or Office Communications Server 2007, Unified Communications Managed API v1.0 Redist.
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[...] have been attributable to either the back end database update not being installed first, or the OCS 2007 R2 .NET Framework 4.0 issue. Having said that, there was at least one report (on this blog) of the “mismatch error” [...]