Prestashop Paypal L_ERRORCODE0 -> 11812

This can happen if you are using non-english characters in your product description.
 
PayPal accepts up to 127 characters for this field and UTF-8 may use up to 6 bytes per character.
 
So the solution is to change the length from 50 to 20 in /modules/paypal/express_checkout/process.php around lines 291 and 310:

//$fields['L_PAYMENTREQUEST_0_DESC'.$index] = Tools::substr(strip_tags($product['description_short']), 0, 50).'...';
$fields['L_PAYMENTREQUEST_0_DESC'.$index] = Tools::substr(strip_tags($product['description_short']), 0, 20).'...';