I know this plugin is no longer updated, but I’m noting this in case anyone has any similar problem: When used with WordPress 5.6, you get an infinite loop that causes WordPress to run out of memory:

/wp-content/plugins/manage-xml-rpc/manage-xml-rpc.php has add_filter('mod_rewrite_rules', 'mxr_htaccess_contents');

Then "function mxr_htaccess_contents( $rules )" calls "$wp_rewrite->flush_rules( false );"

That calls "public function mod_rewrite_rules" in /wp-includes/class-wp-rewrite.php, which in turn calls
$rules = apply_filters( 'mod_rewrite_rules', $rules );

... which calls "function mxr_htaccess_contents" again in the plugin and so on in an infinite loop.

The plugin should not be used with WordPress 5.6.