I've created a empty workflow (just to make sure the error is not in the workflow itself) and when I deploy it from VS 2012 I get this error:
Error1
Error occurred in deployment step 'Activate Features': Microsoft.Workflow.Client.ActivityNotFoundException: The activity named 'WorkflowXaml_89a963f7_e111_4556_a869_e7e93faf9414' from scope '/SharePoint/default/13e35c42-cb8c-4efa-a5a0-d0b2f58f85b1/1e1e2094-774b-44bb-b8f0-c6efd9e59449'
was not found. HTTP headers received from the server - ActivityId: 09339c55-cbdc-4cb3-89f1-4309531e62e0. NodeId: WINGTIPSERVER. Scope: /SharePoint/default/13e35c42-cb8c-4efa-a5a0-d0b2f58f85b1/1e1e2094-774b-44bb-b8f0-c6efd9e59449. Client ActivityId : 8fba226a-9af7-4b70-
00OPCW.SharePoint
Workflow Service Status is OK.
Yesterday everything was working just fine and I can't remember doing anything that would have caused this.