I've set up a Paypal donate button on a client's site. The client tested it; everything worked perfectly. Then I made the mistake of moving the code to another spot on the site. Now, everyone gets the same error: "Sorry your last action could not be completed"

I pasted the code onto a blank page and re-tested with the same result.

I returned to Paypay to generate the code again. This time it gave me a completely different code, but still the same error.

I can't find a solution online. Does anyone have any idea what's causing this?