Originally Posted by ambal
Upgrade of X-Payments to v2.2 should fix the issue, too.
It happens because of line items passed to PayPal from an integrated cart like X-Cart that do not match total of the order. The mismatch happens because of rounding.
X-Payments Hosted users can get upgrade to any new X-Payments version free of charge - just post us a ticket about that!
Please note that this issue was fixed in 2.2 only for the PayPal Pro (Payflow API) gateway.
"Payflow Pro" itself left untouched since it is an universal module (e.g. it may use FDMS instead of PayPal), but this issue is only related to PayPal. We'll consider to change this behavior to avoid such issues in future.
But anyway, If you use PayPal through Payflow you'd better use PayPal Pro (Payflow API) module, since it is absolutely same as Payflow Pro, but supports PayPal-only features (like IPN).