in file acp_search.html, there is a:

<form id="acp_search_index_{backend.NAME}" method="post" action="{U_ACTION}">

				{backend.S_HIDDEN_FIELDS}

				<fieldset class="tabulated">

apparently this is illegal. the hidden fields should be inside the <fieldset ...> tag, not before it.
it parses an "input" tag immediately after the <form ..> tag, which the validator complains about.

RECREATION INSTRUCTIONS:
go to ACP->Maintenance->Search index tag.
validate.