dropdown fields are not working/incomplete

hi,
please take a look here: https://www.backyarddesign.de/_wp2016/produkt/kawasaki/
click at “extras” and check the dropdown fields. whats going wrong?
for this section the plugin “Extra Product Options” is in use. the developers wrote: “…That is probably an incompatibility with the styled select boxes your theme is using. It may be because the options container is hidden at the page load…”
do you have any idea?
regards,
mcihael

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