PayPal USA, Canada error occured during the transaction

[metal-gogo]

Everytime a customer buys something with paypal on my site and then comes back the next error appears:

 

Unfortunately, an error occured during the transaction.

 

Please double-check your credit card details and try again. If you need further assistance, feel free to contact us anytime.

 

(Your Order's Reference: XXXXXXXXXX)

 

 

The purchase is processed.

I have the order in prestashop.

The emails are sent.

Everything looks fine, but the customer believes the payment was not made, and tries it again.

 

 

 

I don't know what could be causing this.

 

My site is in Spanish, but I translated it to English for this question.

 

Any suggestions?

Edited July 16, 2015 by metal-gogo (see edit history)