Nunit3 shows System.AggregateException from an assembly that does not exist with that version

This is the error:

Message: System.AggregateException : One or more errors occurred.
----> System.IO.FileLoadException : Could not load file or assembly 'Microsoft.AspNet.SignalR.Client, Version=2.1.2.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

• We are only running version 2.3.0 of Microsoft.AspNet.SignalR.Client.
• I ran a grep over the whole solution and could not find a single instance which does not use 2.3.0. or runs the version mentioned in the error
• The test itself does not have a reference to this library
• I deleted all bin folders, cleaned and rebuilded.

Update: I am pretty sure the problem is not nunit. Rather it shows the symptons. How do you go about finding out where this reference is used? App.configs for example only show ranges:

<assemblyIdentity name="Microsoft.AspNet.SignalR.Client" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-2.3.0.0" newVersion="2.3.0.0" />

I added an app.config in the test project and in the project that the test is running in which I declared:

<assemblyIdentity name="Microsoft.AspNet.SignalR.Client" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-2.3.0.0" newVersion="2.3.0.0" />

Now the exception is gone. Still I don't think this is the right solution. Normally visual studio must know which assembly version is the right one without declaring an app.config every time.