Fallback handler could not load library on Windows 8

I am making a game in Unity Pro which uses an external C++ DLL.

The HARDWARE_INTERFACE.DLL is wrapped into a class like this:

 class HARDWARE
 {
     [DllImport("HARDWARE_INTERFACE")]
     public static extern bool HARDWARE_Open() ;
 }

The DLL itself is located in Assets/Plugins/.

This is working fine in the editor, and also in a generic Windows build on the development machine which runs Windows 7. In the build, the DLLs are in unity_app/unity_app_Data/Plugins/.

However if I transfer the build to a Windows 8 Desktop machine, the log yields

 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/.\libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 Fallback handler could not load library C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/libC:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
 DllNotFoundException: C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll
   at (wrapper managed-to-native) HARDWARE:HARDWARE_Open ()

It looks as if the whole path to the DLL (`C:/Users/user/Desktop/unity_app/unity_app_Data/Plugins/HARDWARE_INTERFACE.dll`) is being appended to the search paths (`C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/`, C:/Users/user/Desktop/unity_app/unity_app_Data/Mono/lib), which of course is nonsense.

How can I make Unity / Mono find the DLLs on the Windows 8 machine?

Any hints will be appreciated.