One of the problems faced with Prestashop users is configuring the PayPal payment module. After  getting and updating the API credentials from PayPal, you may find that a customer is not able to use this payment option due to internal server error produced when PayPal is chosen as a payment option.

Solution: This is most probably a folder / file permission issue. For some servers, if the folder or file permission is too high it may reject to load the module. Some server consider 777 permissioin as too high. To solve this situation, just change the folder/file permission to 755. It is simple, change any 777 file permission to 755 on the PayPal module folder but leave the ones with 644. The PayPal module folder and files can be found in /modules/PayPal