Caching and the strange contact form problem :(

Hey Friends,

I have a problem that is causing a lot of problems for my business.
For the past 2 months my main contract from on my homepage has not been working and i didn't know because there was no error message - the messages of clients just didn't arrive.

I narrowed down the problem to the caching plugin I was using (FastestCache)
The strange thing is that when I activated the caching plugin initially the Form was working for a while and then sometime in the last months or so it stopped working and I don't know why.

When i clear the cache the form seems to be working fine again (at least on some sites and some browsers) I also cannot exclude it from caching as it is build into the end of every page.

I was told by someone that using a contact form that is build into the theme causes these problems with caching because there is a token that expires and that I have to make sure to activate "autoclearing" of the cache every 6 hours or so but this info. seems a bit shaky to me and I simply can't rely on caching plugins for now which is a true shame.
I really want the speed improvement but not at the risk that my forms don't work.

Also I installed caching plugins for many clients and now i am super worried that their contact forms are not working for all this time and I and they didn't know. I don't know what to do and hope that you can explain to me what is happening and how I can use caching plugins knowing that ALL THE FORMS are working fine ALL THE TIME.

Also what is interesting is that I have another Call to Action Quick From installed using Popup Ninja (you can find it on the left side of the page (sticky) as a red button.

Strangely this Form was working even when the other main form on the bottom of the page was not working anymore. This means that this form was not affected by the cache; but why??

Here's the URL of my Page: https://goo.gl/oLcJ29

Thanks for your Help