Hit an error on Sources/BoardIndex.php found it to be related to using the board viewers mod. The fix was to make one change to the installation routine
change:
<operation>
<search position="replace"><![CDATA[
'new' => empty($row_board['isRead']) && $row_board['posterName'] != '',
'topics' => $row_board['numTopics'],
'posts' => $row_board['numPosts'],
'href' => $scripturl . '?board=' . $row_board['ID_BOARD'] . '.0',
'link' => '<a href="' . $scripturl . '?board=' . $row_board['ID_BOARD'] . '.0">' . $row_board['boardName'] . '</a>'
);
]]></search>
<add><![CDATA[
'new' => empty($row_board['isRead']) && $row_board['posterName'] != '',
'topics' => $row_board['numTopics'],
'posts' => $row_board['numPosts'],
'href' => $modSettings['pretty_root_url'] . '/' . (isset($context['pretty']['board_urls']) ? $context['pretty']['board_urls'][$row_board['ID_BOARD']] : $row_board['ID_BOARD']) . '/0/',
);
$this_category[$row_board['ID_PARENT']]['children'][$row_board['ID_BOARD']]['link'] = '<a href="' . $this_category[$row_board['ID_PARENT']]['children'][$row_board['ID_BOARD']]['href'] . '">' . $row_board['boardName'] . '</a>';
]]></add>
</operation>
to:
<operation>
<search position="replace"><![CDATA[
'href' => $scripturl . '?board=' . $row_board['ID_BOARD'] . '.0',
'link' => '<a href="' . $scripturl . '?board=' . $row_board['ID_BOARD'] . '.0">' . $row_board['boardName'] . '</a>'
);
]]></search>
<add><![CDATA[
'href' => $modSettings['pretty_root_url'] . '/' . (isset($context['pretty']['board_urls']) ? $context['pretty']['board_urls'][$row_board['ID_BOARD']] : $row_board['ID_BOARD']) . '/0/',
);
$this_category[$row_board['ID_PARENT']]['children'][$row_board['ID_BOARD']]['link'] = '<a href="' . $this_category[$row_board['ID_PARENT']]['children'][$row_board['ID_BOARD']]['href'] . '">' . $row_board['boardName'] . '</a>';
]]></add>
</operation>
The string is still unique and what I removed remained uneditted anyway so no real need for it to be included.