Your compiled code is different from the version the runtime is trying to load.
The compilation process is creating an assembly with reference to version Newtonsoft.X
X.dll - but at runtime the bin folder of your website probably contains a different version Newtonsoft.X
Y.dll.
You can make sure the runtime and design time assemblies used are the same - cross check properties of references in your Visual Studio project against the files produced by publishing your website - to figure out why the runtime cannot load up correct version.
Or you can look into adding
Assembly Binding Redirection[
^] instructions in the web.config file.