503: Service Temporarily Unavailable - error message

A new member just contacted me, when they try to login, they get this message:
503: Service Temporarily Unavailable
Too many IP addresses accessing one secure area!
Please contact Support if you require assistance.
I tried resetting their password, but I get the error when I try to login. My "test membership" (not admin login) logs in just fine! I tried deactivating and reactivating the plugin, problem persists!
Can you please advise what to do? Thanks!
David
503: Service Temporarily Unavailable
Too many IP addresses accessing one secure area!
Please contact Support if you require assistance.
I tried resetting their password, but I get the error when I try to login. My "test membership" (not admin login) logs in just fine! I tried deactivating and reactivating the plugin, problem persists!
Can you please advise what to do? Thanks!
David