Cannot Login to make a purchase

I cannot login when I click the "Download" link to make a purchase. I am using my correct email address and password, but I keep getting the same alert: Login has been changed to use email instead of username!

I can login if I click "Login" at the top of the page, but the site logs me out when I try to make a purchase.

Please help. I need to make a purchase.

Same here. This has happened before. Hopefully it will be rectified soon.

see: http://forum.opencart.com/viewtopic.php?f=10&t=158687