We have an old install of Sharepoint 2013 which has the March 2013 PU installed. A bunch of security updates slipped through which patched Sharepoint. You will get the following error:
Method 'Upgrade' in type 'Microsoft.SharePoint.WorkflowServices.WorkflowServiceApplicationProxy' from assembly 'Microsoft.SharePoint.WorkflowServices, Version=126.96.36.199, Culture=neutral, PublicKeyToken=71e9bce111e9429c' does not have an implementation.
This is apparently caused by newer security updates shipping with Sharepoint 2013 SP1 dll files. In order to fix this issue, download and install KB2880963. The install may take some time.