Mixed mode assembly is built against version 'v2.0.50727'

I am using Visual Studio 2012 Ultimate and SQL Server 2012 Enterprise.

Whilst using windows service to execute an SSIS package I receive the following error:

"The Execute method on the task returned error code 0x80131621 (Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information.). The Execute method must succeed, and indicate the result using an \"out\" parameter.\r\n"

Here is my App.config file

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

Firstly, why does this happen?

I have seen various workarounds but this just occurred today and I did not make any changes.