People have experienced some Log-in problems. But it looks like Konami is aware of this Log-in failure.

Translation made by Monika:
Log-in Problems DUEL ACCELERATOR
At this time, customers will not be able to log in to DUEL ACCELERATOR.
We are investigating the situation and working toward resolving the issue.
We sincerely apologize for the inconvenience and thank you for your continued patience.

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Duel Accelerator is now back up, and we are able to log-in, once again!