Opening a JQuery modal window on click of a button with a JQuery plugin

First, make sure that your javascript files are loading correctly. Open Developer Tools while on the page (you can right click and select Inspect).

Under the Elements tab, do a search for your javascript files, to make sure they are being enqueued on the page.

Also check the Console tab for any javascript errors.

Is there a specific reason you’re dequeueing jQuery and enqueuing your own version? WordPress comes with jQuery already included, all you need to do is add it as a dependency to your script file.

Also your registering code needs some things changed:

1. You can do everything in the wp_enqueue_scripts hook for frontend (admin_enqueue_scripts is for backend)
2. When specifying dependencies you should get in the habit of specifying them in an array
3. I recommend using a unique slug for your script/style names, as modal may be registered by other plugins/themes and could be causing issues (i changed to mymodal)

function my_custom_enqueue_scripts() {

    wp_deregister_script( 'jquery' );
    wp_register_script( 'jquery', 'https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js', array(), '3.1.1', true );
    wp_register_script( 'mymodal', get_template_directory_uri() . '/js/jquery.modal.min.js', array( 'jquery' ), '', true );
    wp_register_style( 'mymodal', get_template_directory_uri() . '/jquery.modal.min.css' );
    if ( is_page( 'kundenprofile' ) ) {
        wp_enqueue_script( 'mymodal' );
        wp_enqueue_style( 'mymodal' );
    }
}

add_action( 'wp_enqueue_scripts', 'my_custom_enqueue_scripts' );

You can also change the priority at when your hook is called, by adding it after the function:

add_action( 'wp_enqueue_scripts', 'my_custom_enqueue_scripts', 1 );

10 is the default when you do not specify one

I also recommend JUST FOR TESTING to remove the check with is_page to make it enqueue on every page, and if you do, and it works, but when you put that code back it doesn’t … you know the issue is with that function checking the page (although it’s ALWAYS good dev practices to check the page before enqueueing a script .. i wish all devs did this, but they do not)