I am checking contact page it is ok https://gevcen.be/contact/
Check your forms on a default theme to understand whether the problem caused by our theme. Our theme does not influence the functionality of the plugins.
I am not checking some default theme, because my site is live !! Seriously!
I am using your theme, that’s all.
Now please check again the following link, there is an easy way to get the error message it’s by trying to send twice a form (1st time is OK, but 2nd is refused to avoid spam).