Hi, i had this issue on three different Prestashop website, all with the latest version of prestashop. The issue is very simple, after some time, the module Paypal disappear from front-end and users are not enable to use it. configuration was right, but simple module doesn't works.
The only way to restore it, is to uninstall module and to reinstall it.
But after few time, it will happen again, and module will disappear again.
This is a bug of prestashop, i think.
Please solve, thanks