That error is largely ignorable. It simply means I build Rhythm towards the lower RevitAPI dlls than you are currently working in.
From a Revit developer point of view, I typically build a version of the Addin for each version of Revit. But with Dynamo, managing a version of Rhythm for every version of Revit is ridiculous.
Unfortunately, I don’t think you will ever see no errors in that Dynamo window because of the way it was built.