account sign in error - "E-Mail has been used"

I'm trying to sign into airdroid.com using my Google account. It prompts me to sign up for an AirDroid account and gives me the dialog options shown in the attachment.
Am I missing something? What does "E-Mail has been used." mean? Of course it's been used, I just signed in using it.

Any help would be appreciated.

Hi TWillloughby,
It seems a rare issue. Would you please post the screenshot again so that we can figure out the cause?

Why Does this Happen??

Hello Kalhey,
You got the same error message?

Would you please sign in with that email address? And if you forgot your password, you may reset it here: https://id.airdroid.com/p7/user/sendresetpwmail.html