Why I'm getting PayPal payment error?

My customer got an error "Paypal cannot process this transaction because of a problem with the seller's website. Please contact  the seller directly to resolve this problem". However when the customer tried to purchase less quantity it went through. Can't understand why it happened, and how it won't happen again.

Thank you for contacting our Support.

Please be mindful that everything related to payment is happening on the payment processor's side, so we have no control over it. I checked your PayPal integration and everything seems fine, so I'm afraid that you will have to contact PayPal support to check what happened.

If you still experiencing problems please let us know.

Thank you!