Trying to use the EC2 plugin, after I add the keys the region list remains empty. Then when I add a private key and test the connection I get the following error:
The EC2 key pair private key isn't registered to this EC2 region.

I'm assuming this error is due to the fact that ec2-plugin picks a default region?

Also when I check the console of the browser I find:
http://*.*.*.*:****/descriptorByName/hudson.plugins.ec2.AmazonEC2Cloud/fillRegionItems Failed to load resource: the server responded with a status of 500 (Server Error)

Could it be that being behind the proxy causes this problem?