Warning CS1685 as Error

I am getting warning CS1685 as Error (can not compile/run)

 warning CS1685: The predefined type `System.Runtime.CompilerServices.ExtensionAttribute' is ambiguous. Using definition from `System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'

Details of the message reference two files:

 C:\\Program Files\Unity 5\Editor\Data\Mono\lib\mono\unity\System.Core.dll (Location of the symbol related to previous warning)
 C:\Users\Roman\Documents\savit-unity\SourcesUnity\Savit.Unity\Assets\Plugins\x86_64\Microsoft.Scripting.ExtensionAttribute.dll (Location of the symbol related to previous warning)

And unhandled exception:

 Unhandled Exception: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.
 at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
 at System.Reflection.Assembly.GetTypes () [0x00000] in <filename unknown>:0 
 at Mono.CSharp.RootNamespace.ComputeNamespaces (System.Reflection.Assembly assembly, System.Type extensionType) [0x00000] in <filename unknown>:0 
 at Mono.CSharp.RootNamespace.ComputeNamespace (Mono.CSharp.CompilerContext ctx, System.Type extensionType) [0x00000] in <filename unknown>:0 
 at Mono.CSharp.GlobalRootNamespace.ComputeNamespaces (Mono.CSharp.CompilerContext ctx) [0x00000] in <filename unknown>:0 
 at Mono.CSharp.Driver.LoadReferences () [0x00000] in <filename unknown>:0 
 at Mono.CSharp.Driver.Compile () [0x00000] in <filename unknown>:0 
 at Mono.CSharp.Driver.Main (System.String[] args) [0x00000] in <filename unknown>:0 

Last line is:

 The class System.Configuration.ApplicationSettingsBase could not be loaded, used in System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

It should be only compiler warning so what does it Unity treat it like an error? More importantly what can I do about this error?

Thank you