HRESULT 0x8007006E on including a DLL into a project 'lib' folder

For a project I am working on, my team generates reports on the server with ActiveReports. We've generated a License DLL that allows report functionality to work.

In our VS2012 solution, we've included a /lib/ directory that holds certain external DLLs, such as the ActiveReports assemblies and this license DLL.

However, upon attempting to add the DLL to the project, we get two errors, back to back. I attempted to add the extant DLL using these steps:

1. In VS 2012, right click our /lib/ folder, and click 'Add Existing Item...'
2. Browse to the Windows directory where the License DLL Is and select it, click Open.

Upon doing this, though, I receive two errors back to back:

1. Could not open the file
2. The system cannot open the device or file specified. (Exception from HRESULT: 0x8007006E)

I have checked my local operating system permissions to ensure that the folder the DLL lives in is accessible, and that the path of the object that appears in VS 2012 points to the correct place. Question: What is causing this error? Surely, I'm missing something...

I received the same error trying to open a file with VS's binary editor. The file was already open in another app. After closing the other app and I was able to open the file in VS without error.