IntrospectionExtensions error despite targeting .NET 4

I have an application that is crashing when running on a .NET 4 server with the following error:

Could not load type 'System.Reflection.IntrospectionExtensions' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

I realise that this means that I need to install .NET 4.5 on the server but sadly this is not possible due to disc space constraints.

I am building this project in VS15 with .NET 4.6.1 installed.

As far as I can tell, there are no parts of my code that are using Reflection and consequently I'm confused as to why I'm getting this error. Especially given that I am targetting the .NET Framwork 4.

I have also tried including the Mono version of IntrospectionExtensions nabbed from some other site in my project but this was just generating lots of unresolvable symbol errors.

Is there any way to get around this issue?

I have attached my code below for reference:

I have uploaded it to Hastebin as there's no point copying 600 lines of code here when I don't know which section is causing the problem.