The assembly 'XXXX' could be any assembly. This exception is thrown when your application finds an assembly that does not match what you are looking for. A very common mismatch is version number. For example, if you have added a reference of any dll with version 18.104.22.168 but in your web config it is referencing to different version (say 22.214.171.124), you will get this error.
One thing to note that for simply named assemblies version number is ignored but for strongly named assemblies version number is not ignored.
You will also get this error if you have PublicKeyToken mismatch. To check PublicKeyToken of your referenced assembly, you can run the following command in your Visual Studio Command Prompt (point to dll's folder),
sn -T DLL_Name.dll (This will only work if your assembly is strongly named).
You can either manually update your web.config or you can delete all the assemblies and associated files from the project bin folder and add them again one by one. After Recompiling it will fix your problem.
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