Ok, I found this piece of code in my functions.php file:
function deregister_qjuery() {
if ( !is_admin() ) {
wp_deregister_script(‘jquery’);
}
}
add_action(‘wp_enqueue_scripts’, ‘deregister_qjuery’);
I commented it out and paypal buttons are now showing.
I don’t remember I did this, it’s probably some plugin did that. Whether I deactivated all plugins and change the default theme, this piece of code remains inside functions.php file.
Thanks to Niklas that pointed out javascript make error I was able to find this.
The problem is solved.