I am facing the same problem as seen in this http://forge.prestashop.com/browse/PNM-1165
It says this following message after I click on 'pay with paypal'
Please try to contact the merchant:
I don't know what to do T^T
Im using prestashop 1.5.4 and paypal module 3.5
There is one reply suggestion from Francois Gaillard added a comment - 29/Mar/13 9:29 AM
He said that
This issue occurs when your server IP has been blocked by the PayPal's API. If you are running share hosting environment the server can be banned blacklisted by PayPal.
Please, try to contact PayPal support they should be able to help you.
Let us know when you have more details please.
That answer is incorrect in as much that we do not block IP addresses to our API endpoints without making the merchant aware.
It is correct, from our experience, that the above error occurs in PrestaShop when your server is unable to establish a successful HTTPS connection with our API endpoint.
I suggest you take a look at General troubleshooting - API timeout and run the tests provided in it.
Pay special attention to the AT&T block we mention in there, as we've seen a large number of API callers getting blocked at that level. Unfortunately that block is beyond our control - but we've heard good reports of merchants getting in touch with AT&T and getting that block lifted (if confirmed to be blocked there).