Frontpage not working after several minutes

At first, my english isn't very well, so i hope you will understand.

We have copied an existing zencart to a subdomain, to do some changes (added dynamic filter etc.) no problems there.
Now we copied this zencart back to the original location.
Everything works fine so far.

But after about 15 minutes inactivity in the front or backend, the front-end does not work anymore.
When you login in the back-end, the frontpage works again, untill about 15 minutes of inactivity, then it stops working again.

Does anyone know what the problem can be?

We had this before on an other website, but forgot what to do to get it fixed. I only remember it wasnt that difficult to fix.
Can anyone please help?

Post a link to the frontend?

https://www.legarcon.nl thats the frontend.

But... it works now, cause we are active in the backend

Try switching to the classic template and see what happens
If it does not do the same then you have narrowed it to some template file or files (read addon) that is only in the custom template

We dont have anything changed in php files, stylesheets etc.
Thats not the problem.

Busted cookies perhaps? Clear your browser's cache and cookies, then quit your brower (all its windows/tabs), and restart it.

We are working on 6 different computers here...
Its not a problem of our computers......

Doesnt mind where you are, here, in tokio, usa, africa, or on what computer, phone or whatever.
The frontend stops working after about 15 minutes not clicking on frontend and/or backend.

We have our own VPS.

Problem now (so you can see what the problem is) is that the webshow owner is constantly clicking around in the backend. (some sort of **** cow)
We've asked here to log out and stop... now we will wait.....

We dont have anything changed in php files, stylesheets etc.
Thats not the problem.
These are not part of a default ZenCart install so you have added some things
dynamic_filter
cookie_control
src="http://tools.allebedrijvenin.nl/kwaliteitsmeter/kwaliteitsmeter.js

<script src="http://www.geoplugin.net/javascript.gp

Haal eerst maar eens even al die cookie code weg en kijk dan of het je nog problemen geeft. Zo niet dan is er wat anders aan de hand, zowel dan is er iets mis met de configuratie van je cookie code. In de header geeft de code aan om alleen aan Britse bezoekers de cookie waarschuwing te laten zien, maar helemaal onderaan geeft de code aan om het alleen aan Nederlandse en bezoekers van Belgie te tonen, dus de configuratie kan ook een oorzaak zijn.

Hier in de VS krijgen we geen waarschuwing terwijl andere Nederlandse websites dat wel doen.