Unsupported Back-End Version for a .NET Search Interface
The CES version for the specified server is not compatible with the Coveo .NET Front-End version that you are configuring. CES version X.X.XXXX or higher is required.
You upgraded Coveo .NET Front-End and your Coveo Enterprise Search does not support new Front-End features.
You installed a new Coveo .NET Front-End version and bypassed the installation warning notifying you that your Coveo Enterprise Search was too old. Your Back-End does not support new Front-End features.
On your Coveo Master server , upgrade your Coveo Enterprise Search (see Upgrading CES).