If you install all the current patches to your server before installing OCS it will install .NET Framework 4.0. When .NET 4.0 is installed OCS cannot detect .NET 3.5 and you receive the error message
Microsoft Office Communications Server 2007 R2, Microsoft Unified Communications Managed API 2.0 Core Redist 64-bit installation requires Microsoft .NET Framework version 3.5. Installation cannot continue.
Microsoft has put up a knowledge base article kb2224981 describing the issue and also the work around.
To resolve this problem, follow these steps:
- Remove .NET Framework 4.0 from the server.
- Install the OCS products that you want to have on the server
- Install .NET Framework 4.0 on the server