Validation doesn't work

I have a problem on my site with the validation since a couple of weeks.

The javascript for validation gets called. However since fm_registered_form_items always contains 0 items, it surpasses all the checks and submits anyway.

Am I doing something wrong? I’ve had it working for quite some time, and now it doesn’t validate anymore.

http://www.ho-modelautoclub.nl/ruilbeurs/deelname-ruilbeurs/

https://ww.wp.xz.cn/plugins/wordpress-form-manager/