Exception with regards to SharePoint Workflow Services

Hi there,

Since today we suddenly get the following exception on our development farm:

Method 'get_IsClientValid' in type 'Microsoft.SharePoint.WorkflowServices.WorkflowServiceApplicationProxy' from assembly 'Microsoft.SharePoint.WorkflowServices, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' does not have an implementation. 

We get this exception when we:

- Open up a site

- Open up Manage Service Applications from Central Administration

We don't get the exception when I open up the starting page of Central Administration.

Unfortunately we don't know what we have done that has resulted into this exception.

What is the root cause and is this easy to fix?