"System.ServiceModel.Primitives"- assembly not found or dependecy not found

I Have a project with a ASP.Net Web api. Since I added a Method to upload and download files a get a strange error.

The file or assembly "System.ServiceModel.Primitives" or a dependency thereof was not found. The signature with a strong name could not be verified. The assembly may have been modified or delayed, but not fully signed with the correct private key. (Exception of HRESULT: 0x80131045)

If I get a knew copy of the repo the error doesn't occurs for about an hour. Can anybody help?

Please check ASP.NET Web api project reference System.ServiceModel.Primitives dll. please remove the reference then add and re-build solution and try.