This issue has many causes... in my case the problem was that in my web.configweb.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<add assembly> tag from my web.config resolved the issue.