When I use the Donate Plus plugin, I get an HTTP 500 Internal Server Error when I use the "live" setting. When I use the simulator (sandbox), everything runs just fine.
The problem seems to be on Paypal's side, but then again it would be down for more than 6 hours already. So now I'm thinking there is something wrong with the plugin. But what?
You can try to see what happens over here: http://www.kwestievan.nl/unityexpress/donate
It happens a few moments after you press the Donate button, so don't worry about actually donating money. You can stop after you have reached the login page for PayPal.
Anybody having an idea?