Searching redirects to the home page

Homepage? https://www.domain.tld or sub.domain.tld? I am gonna take a shot in the dark that what you may be seeing is because of your theme (Hint: knowing what theme you are using so someone could reproduce your result would be helpful).

It is or was common to write the seachform.php in a theme using

action="<?php echo $_SERVER['PHP_SELF']; ?>

In multisite this may return you to the https://www.domain.tld/index.php – which would be fine in a single WP install of course.

The fix, if you have that code indeed in your theme, is to change it to

action="<?php bloginfo('home'); ?>

Hi and thank you very much for your response!

The problem is occurred only in one subdomain. So searching it redirects in sub.domain.tld

Furthermore this ain’t a theme issue as it is occurred in any theme (default etc).

The theme’ searchform.php file is like this:

<?php do_action( 'bp_before_blog_search_form' ); ?>

<div id="top-search-container">
<div id="adv-search-link"><a href="<?php echo home_url(); ?>/searching"><img src="<?php echo get_template_directory_uri(); ?>/images/adv-search.png" alt="Advanced Search" /></a></div>
<form method="get" id="searchform" action="<?php echo home_url(); ?>/">
<div id="searchform-child">
	<input type="text" title="<?php echo of_get_option('t-8', 'Search' ); ?>" value="<?php the_search_query(); ?>" name="s" id="s" class="hoverText hoverTextActive" />
	<input type="submit" id="searchsubmit" value=" " />
	<?php do_action( 'bp_blog_search_form' ); ?>
</div>
</form>
</div>

<?php do_action( 'bp_after_blog_search_form' ); ?>

I just noticed that if I change the permalinks from post name to default, searching works properly.

But I need pretty permalinks not the default.