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?