JSF partial response shown plain text on every reload

Question

What can cause this message :

<partial-response id="j_id__v_0">
    <changes>
        <update id="j_id__v_0:javax.faces.ViewState:1">i+eTRcNJKFa/X70WEJFydAaAltIQr95KirDWpXlROBzc6LqS</update>
    </changes>
</partial-response>

It appears every time I reload the page.