C# .DLL not recognized on only one unity installation

Hi Guys. This is a real headscratcher to me. I have a dll file in Unity (of the YamlDotNet library). It works fine on about 7 different machines, all running Unity 2017.1.0f3 or the previous version (Unity 5.something). Now I have a new MacbookPro running OSX 10.12.5 and the same Unity version and Unity throws this error:

Assets/Scripts/steelCircus/Editor/InputEditor.cs(4,7): error CS0246: The type or namespace name `UnityInputConverter’ could not be found. Are you missing an assembly reference?
(UnityInputConverter is the namespace of the .dll)

To reiterate: There is absolutely no Problem on my other MacbookPro running OSX 10.11.6 and Unity 2017.1.0f3 (the same Unity version).

Both installations seem to be 64 bit. Using Terminal and running “file” command on the “Unity” file inside the MacOS folder of the package yields: “Mach-O 64-bit executable x86_64” in both cases. I tried moving the dll into the root “Asset” folder into the “Asset/Plugins” folder into the folder with the c# file that uses it, out of the folder. Nothing has any effect.
Trying to manually add the DLL to the “.NET Assembly” tab of MonoDevelop yields the message:
“File: ‘Blah/Blah/UnityInputConverter.dll’ is not a valid .NET Assembly”

If anyone has any ideas what I can try, I’m happy to hear them. Thanks.

I guess you talk about this DLL? Have you compiled the DLL yourself or have you downloaded it somewhere? When MonoDevelop complains that it’s not a valid .NET assembly the file might be corrupted.

Also note that the DLL seems to be an editor extension (as it references the UnityEditor.dll) so the DLL must be inside an editor folder.