My paypal sandbox test account is not working (www.sandbox.paypal.com), it is recursively asking for login even if i am already loggedin on developer.paypal.com.
image url: http://postimage.org/image/i3bd584uj/
It was working fine till 6th march.
Strange as it may seem, deleting your developer.paypal.com should resolve this. The reason for this is that you've got cookies from the old developer portal in place which interferes with the new portal / sandbox environment. After you've deleted all your cookies / all your paypal.com cookies, you should be able to login to the new developer portal and access any sandbox accounts without any issue.
A fix for this is going out shortly.