Paid paypal order, but no order visible

I have had this problem twice, so it doesn't happen with all orders. I don't know where to start debugging or searching a solution.
I have had PAID paypal orders, but none of these orders are listed in the Customer/Order listing. This means I have to contact the customer and ask them about what they actually ordered.
In the extension/payment/paypal option, Test mode is disabled, Transaction method is set to Sale, Order status is set to Processing and this option is enabled with priority 1 for all geo zones.

What can be wrong?

oz457 wrote:I don't know where to start debugging or searching a solution.

How about the other 50 threads on the same topic here in the forums? That's probably the best place to start.

What version of OpenCart?
Have you tried this Solution?
Or even better, this solution?