drop down menu’s, checkboxes and buttons not registered as valid answer

Dear,

The contact forms on my website are experiencing some trouble.
At first, the contact form looks fine, but as soon as you want to send it, it stays on loading forever.

In the back-end, some data is registered, which is all the data up to the first questions including either drop-down menu’s, check boxes, or any other selection box. These somehow not ‘recognized’ as a valid answer. This results in the form not uploading properly, and not displaying the data, when wanting to see it in Contact Form DB.

Via the following link (https://www.svflow.nl/en/TEST/), you can go to a TEST page I made, with an example of one of the contact form, along with some other information on the plugins and the theme used. Good to know, is that everything is updated to it latest version.

Perhaps you experienced this as well once, and have the solution to my problem, or have something similar going on.

The page I need help with: [log in to see the link]