Hey, I have found something more:
Sync workflow failed with error message - Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=7.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: An unexpected error occurred.Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
<ErrorCode>-2147220970</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>An unexpected error occurred.</Message>
<Timestamp>2015-05-04T09:20:28.0569221Z</Timestamp>
<InnerFault>
<ErrorCode>-2147220970</ErrorCode>
<ErrorDetails xmlns:d3p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>System.InvalidCastException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #985CD8C6</Message>
<Timestamp>2015-05-04T09:20:28.0569221Z</Timestamp>
<InnerFault i:nil="true" />
<TraceText i:nil="true" />
</InnerFault>
<TraceText i:nil="true" />
</OrganizationServiceFault>
at System.Activities.WorkflowApplication.Invoke(Activity activity, IDictionary`2 inputs, WorkflowInstanceExtensionManager extensions, TimeSpan timeout)
at System.Activities.WorkflowInvoker.Invoke(Activity workflow, IDictionary`2 inputs, TimeSpan timeout, WorkflowInstanceExtensionManager extensions)
at Microsoft.Crm.Workflow.SynchronousRuntime.SynchronousWorkflowActivityHost.ExecuteWorkflowUsingInvoker(Activity workflow, ICommonWorkflowContext context)
at Microsoft.Crm.Workflow.SynchronousRuntime.SynchronousWorkflowActivityHost.StartWorkflow(WorkflowActivationData activationData, ICommonWorkflowContext context)
, error code - -2147220970
So and this is the overview:

So it already fails at the Create step..
But the ContactReference I'm entering as Target Parameter vor the Custom Action is fine.