Hi,
After reading your post, I logged into my personal account via the website and had no issues.
Also saw that the diamond sale is still active.
Your "failed attempt" message could be attributed to a couple of things:
1) Wrong credentials (account email and password)
2) Having more than one account attached to the same email address.
If #2 is indeed the case. Please read
this post by Majik as to how to get you able to log in.
~ Beren