Paypal have recently changed their user interface in developers.paypal and as such, I couldn't find out the place from where I can log in to my old SANDBOX account.
In the new developer.paypal.com, on the Application tab, I selected the sandbox account,
In the following link,
https://developer.paypal.com/webapps/developer/applications/accounts
I get a button "login with paypal". But when I try to login, it displays a message "Invalid email" even though my email and password are totally correct.