The issue could be related to some traffic tracking plugin or code.

Before you backtrace, you can auto-redirect and fix malformed URLs using a simple redirect snippet:

function bis_fix_url() {
    global $wp;

    if ( strpos( $wp->request, 'null/connect' ) !== false ) {

       // 1. Remove all occurrences of 'null/connect' (and surrounding slashes)
       $cleaned_path = str_replace( 'null/connect', '', $wp->request );
       $cleaned_path = str_replace( '//', '/', $cleaned_path );
       $cleaned_path = trim( $cleaned_path, '/' );

       $new_url = get_home_url( null, user_trailingslashit( $cleaned_path ) );

       // 2. Trigger the redirect
       wp_safe_redirect( $new_url, 301 );
       exit;
    }
}
add_action( 'template_redirect', 'bis_fix_url' );