TypeLoadException after adding DLL

Hey, I'm facing following problem:

After adding several DLL's to my project (Unity 3.5.7 Pro), I get following error:

 TypeLoadException: Could not load type 'UnityEngine.MeshSubsetCombineUtility+MeshInstance[]' from assembly 'UnityEngine, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
 UnityEngine.InternalStaticBatchingUtility.Combine (UnityEngine.GameObject[] gos, UnityEngine.GameObject staticBatchRoot, Boolean generateTriangleStrips) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/Export/StaticBatching/CombineForStaticBatching.cs:137)
 UnityEngine.InternalStaticBatchingUtility.Combine (UnityEngine.GameObject staticBatchRoot, Boolean combineOnlyStatic, Boolean generateTriangleStrips) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/Export/StaticBatching/CombineForStaticBatching.cs:48)
 UnityEditor.PostprocessScene+UnityBuildPostprocessor.OnPostprocessScene () (at C:/BuildAgent/work/14194e8ce88cdf47/Editor/Mono/PostprocessScene.cs:22)

without even referencing the DLL's in code.

I tracked the problem down to on DLL ( NationalInstruments.DAQmx.dll ).

Did anyone have any similar problems regarding UnityEngine.MeshSubsetCombineUtility?

Best regards, Sven