very rare ''captha not valid '' error

Good afternoon.

Today, i launched a giveaway on the website and to get some entries, people would need to make an account on the website itself.

For some reason, there is one user that cannot access her account, no matter what. Captha says its invalid as she would be some kind of spammer. She tried this on the opera browser, firefox and chrome. I can also see that other users are not able to login for some time to time.

I really dont know whats happening, i dont see anything bad in the console as well nothing in the chrome developer toolkit. i already deleted some old plugins i ain’t using anymore as well for disable and enable of the google captha.

Could she be using a TOR network? or a VPN? The ip looks normal to me.