This issue has many causes... in my case the problem was that in my web.config
a tag adding the System.Runtime
assembly:
<assemblies>
<add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</assemblies>
but one package also added the same assembly as dependency with other version:
<package id="System.Runtime" version="4.3.0" targetFramework="net47" />
removing the <add assembly>
tag from my web.config
resolved the issue.