Paypal express works for me, Paypal IPN doesnt

Hello,

Sinds the beginning of my ZenCart 1.3.8a installation i'm having trouble with the Paypal IPN module.

I get multiple orders, one empty and one with products.
Lately im receiving only the empty order...no products in it.

You might think its because of a slow server respons, but I host a differend website on the same server which doesnt have any Paypal issues.

Even better, while testing Zen's Paypal Express module, that one seems to work just fine.
I could use the Express module, but then I get the express buttons all over the place, and thats what I don't want because I need my customers to follow the complete checkout.

Help will be very much appriciated.
If more info on this subject is needed, please ask.

Thanks!

Now while making a payment with Paypal Express, I receive the following message:

10413 Transaction refused because of an invalid argument. See additional error messages for details. - The totals of the cart item amounts do not match order amounts.

I re-checked all the Paypal files in the Zen installation, they are all equall to a fresh Zen installation.
Maybe im missing any updates?