Invalid Invitation code from a correct code

[nkhayet]

Looks like there is a problem happening to a lot of people in the last 2 days that they are receiving an invitation code on the email and when trying to activate it shows this error message: "You've entered an invalid code, or one that's already been claimed. Please check your code and try again".
I have no idea why this is happening.. If anyone can explain a little bit about the invitation system and why this problem is occurring?

[jobevers]

Closing this as its old; please re-open if it still persists on the most recent version