This is the form definition that I see on the page:

<form action="http://beta.agro-chemie.nl/zoeken/" method="post" class="searchandfilter" data-sf-form-id="6965" data-is-rtl="0" data-results-url="http://beta.agro-chemie.nl/zoeken/" data-ajax-form-url="http://beta.agro-chemie.nl/?sfid=6965&sf_action=get_form" data-use-history-api="1" data-template-loaded="1" data-lang-code="" data-ajax="0" data-auto-update="1" id="search-filter-form-6965" autocomplete="off"><ul><li class="sf-field-search" data-sf-field-name="search" data-sf-field-type="search" data-sf-field-input-type="">

And it is missing a lot of variables.

What version of the plugin are you using? It looks like it is 2.1.x, which is VERY old. Your license for support and updates has expired. We are currently at 2.3.4 and shortly will release 2.4.0

You need to renew your license (after renewal, it may take a while before the server updates your account) to be able to download the latest version from your account area) and receive further support. To update, you will need to use this method:

1) Deactivate S&F Pro in WP Admin Plugins page (do NOT delete the plugin from within WP Admin Plugins page; otherwise you will lose your search form)
2) Via FTP (or cpanel file manager if you have it), delete search-filter-pro folder from the wp-content\plugins folder on your server. If you wish to keep the folder, just in case, do NOT rename it, instead move it UP a folder into the wp-content folder.
3) Unzip the file that you have downloaded (if you are using cPanel File manager, you can upload the zip file and then extract it in the plugins folder).
4) Upload the search-filter-pro folder to the wp-content\plugins folder on your server.
5) Activate Search & Filter Pro in WP Admin Plugins page.