We have been experiencing the same problem for the last day or more. (Our site is a small "hand-built" site supporting a small number of users -- so it might have been happening longer and was not reported; our last payment was received Sept 24.) Our site simply posts a form to https://www.paypal.com/cgi-bin/webscr. We pass hidden fields in the form, containing the individual items in the "cart". This approach has been working great for several years, and now suddenly I got reports from users of the dreaded message: "Things don't appear to be working at the moment. Please try again later." I tried it in our "development environment" (still posting the form to the production PayPal server) and sure enough, I got the error page. I then examined the redirect URL that PayPal had produced, to send my browser to that error page. And at the end of it, I saw "AMOUNT_ERROR". I thought that was odd. I carefully examined the fields in the form that I was posting, and confirmed that the amount was there, just like always. PURELY ON A HUNCH, I thought, "Maybe PayPal made a change on their end, in the way that they parse the amount fields." Up until now, we had been formatting the amounts as US currency (for example: $140.00). Could it be that PayPal made a change and can no longer handle the $ symbol in the amount field? So, I changed the application code to produce the amount fields as "raw" numerical values, without any currency formatting. And it worked. No more error.
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