"Your payment session has expired..."

"Your payment session has expired..."

I've had a few customers report that they receive this error when trying to pay my invoices:

Your payment session has expired. Please return to the participating website and try again.

I've started checking the link in the generated e-mail before sending the invoice and today, I got it myself.  I deleted the invoice and recreated it, but I still received the error.  Then, I logged into my PayPal account and tried the link again and it worked. It's easy enough just to tell a customer to log into PayPal and then click the link, but it works for most, so it seems like there should be a solution.  Any ideas?