Hi, Johannes.

1. Let's try this first, ( the easy method)  and see if it solves it.

Please go in the main plugin settings, and uncheck/disable "Include full bootstrap front-end".

Then refresh the page, let me know if the dropdowns start working?

2. If not - then it might be a conflict with something on site,

like the current Theme  ( since you already tried disabling other plugins, we will rule that one out);

Can you try to change your Theme, and refresh the page - see if the issue is gone?

If it is, then we will see what to do next.

If you still can't isolate this issue, we can remotely log in and see what we can do.

Please provide me a temporary WP-admin (administrator) user for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. 

We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course, we do not provide login data to third parties. 

You can write credentials here just check PRIVATE Reply so nobody can see them except us.

Hi, Johannes.

Firstly, my apology for the waiting time on this reply.
We had our Orthodox Easter holiday break which increased the delay.
Thank you very much for your patience.

-

Thank you for sending this error text, and you are absolutely correct :

'Uncaught TypeError: Bootstrap dropdown require Popper.js'

It is exactly like that, at the moment we have this issue, our plugin has conflicts if you use Bootstrap higher than 3.

There are a couple of users who managed a custom workaround to get our tables working with Bootstrap 4, though.

-

Our developers are doing their best to upgrade our Bootstrap version as soon as possible;

i can't promise an ETA when it will be finished, though.

You can certainly follow our changeLog page if you'd like ( it is also available in the plugin dashboard), where we state any changes/new features/bug fixes during updates;

and our newsletter, so you're informed about new features, bug fixes, freebies, etc.

-

For the moment, i will share the workaround with you, you can try to see if it helps.

Please go to the Customize section in the table settings, and in the Custom CSS section add this: 

.wpDataTablesFilter[data-wpdatatable_id="8"] .dropdown-toggle::after {
    display:none !important;
}
.wpDataTableID-8 .dataTables_length .length_menu.show{
    display:inline-block !important;
}
.wpdt-c .bootstrap-select > .btn-default:after {
    display:none !important;
}

( Change the table ID to your table ID)

Then, in the functions.php file of your child theme add this:

function bootstrap_dequeue_script() {
    $page_content = get_the_content();
    $pageContainsWpdt = has_shortcode($page_content, 'wpdatatable');
    if ($pageContainsWpdt){
        wp_dequeue_script('bootstrap-js');
        wp_enqueue_script( 'bootstrap-popper', 'https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.0.4/popper.js', array('jquery'), null, true );
        wp_enqueue_script( 'bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array('jquery','bootstrap-popper'), '3.3.5', true );
    }
}
add_action( 'wp_print_scripts', 'bootstrap_dequeue_script', 100 );

Add this to your active Theme, 

or child Theme, and let me know how it goes.

Thank you.