Sometimes the "<providers>" configuration is not removed while downgrading from EF-6 to EF-5.
some simply remove these "<providers>" tags inside the <entityFramework> tag.
<entityFramework>
<defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
<parameters>
<parameter value="v12.0" />
</parameters>
</defaultConnectionFactory>
<!--<providers>
<provider invariantName="System.Data.SqlClient" type="System.Data...