Express Checkout Digital Goods : Proxy Error on sandbox.paypal.com/incontext

I have a Flash website. When I want to use Paypal Express Checkout with Digital Goods, I call this javascript code :

dg = new PAYPAL.apps.DGFlow();
dg.startFlow("http://mydomain.com/setup.php");

setup.php calls SetExpressCheckoutPayment function and redirect to https://www.sandbox.paypal.com/incontext?token=...&useraction=commit

With Firebug I can see this address returns a 302, and redirects to https://www.sandbox.paypal.com/webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow?exp_type=&cookiesBlocked=&token=...&useraction=commit This adress returns also a 302 and redirects to https://www.sandbox.paypal.com/webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow?execution=e1s1

Here it hangs for several minutes and ends with this error message :

Proxy Error
The proxy server could not handle the request GET /webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow.
Reason: Error during SSL Handshake with remote server

I started to get this error sometimes last week, and I have it every time today. It happens on my MAMP environment and on my website. I don't have SSL certificate but I didn't last week and it was not a problem.

Do you know anything about this error message ?

Edit

I tried with Opera, proxyError comes at a different step : https://www.sandbox.paypal.com/webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow?execution=e1s4

And once this morning on Firefox I had another Proxy Error after the first redirection :

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow.
Reason: Error reading from remote server

Solution

I don't have the Proxy Error anymore since yesterday. I didn't change anything so it seems PayPal servers are unstable...