This has been reported as occurring after updating Microsoft Excel on a PC running Unity 11.0.
The update of Microsoft Excel to the latest version also updated the '.NET Framework' libraries that were installed on the PC.
The issue could be caused by the update of '.NET Framework' replacing some of the .NET Framework 3.5 components.

-  Perform a <Repair> off the installation DVD.

-  Reinstall the Microsoft .NET Framework libraries used by Unity Pro.
To do this:
Open the Unity Pro installation DVD.
Open the folder named 'ISSetupPrerequisites'
Open the folder named '{074EE22F-2485-4FED-83D1-AAC36C3D9ED0}'
Double Click on the fie named 'dotnetfx3.5.exe'

-  Upgrade to Unity Pro v11.1

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