In 4.5, the implementation of XmlSerializer was replaced with one that isn’t dependent on the C# compiler. While it provides better startup performance and stability, you might be running into a compatibility issue between the implementations. Can you try adding the following to your app.config file and see if that fixes the issue?
<configuration>
<system.xml.serialization>
<xmlSerializer useLegacySerializerGeneration="true"/>
</system.xml.serialization>
</configuration>
If you’re concerned about having this work on 4.0, you could try detecting the version of the framework at runtime, and dynamically change the configuration if the runtime is 4.5 or higher. I wrote a blog post a while back explaining how to do that: