Thank you! That “Older Posts” button is indeed generated by Infinite Scroll. The code you posted above will help you change that text. I just tested it again, and it works well.
If the code breaks your site, try to insert only the following in a functionality plugin like this one:
function jeherve_custom_infinite_more() {
if ( is_home() || is_archive() ) { ?>
<script type="text/javascript">
//<![CDATA[
infiniteScroll.settings.text = "Custom Text";
//]]>
</script>
<?php }
}
add_action( 'wp_footer', 'jeherve_custom_infinite_more', 3 );
I hope this helps.