Slider plugin interfering with simple self-made contact form

Hello ??

I created a simple contact form

https://pastehtml.com/view/198epwq.txt

that uses jquery for form validation.

https://pastehtml.com/view/198eqfd.txt

The form works fine but after I added the WP Featured Content Slider, https://www.iwebix.de/featured-content-slider-wordpress-plugin/

the form validation no longer works. When I submit it without inputting text in the fields it loads the sendmail.php and shows number 3. When I send it successfully it shows number 1.

I know the slider content is causing this but I have no idea why. Can someone please help me? I cannot even imagine a solution for this.