Pro Sites testing - sandbox paypal returns blank page

Hi there,

I'm trying to build multisite using ProSite. I've registered business and personal sandbox accounts for Paypal, entered sandbox API credentials in the ProSites settings panel, Changed IPN address to the one I had in the settings page. However, when I try to pay for the premium level I only get a blank page in return.

You can register at stronademo.waw.pl/wp-signup.php and see for yourself.

Any idea why it's happening?

Edit. I've just checked that when I switch to Live Mode with sandbox credentials I get redirected just fine. Odd.