I am having the same issue as akiraanastasia.
This just started a couple of days ago and now I am blocked from my site.
We have a couple of users accounts for different people. These are people from different parts of the world. ALL of us are getting the same message even we have different accounts to log into our website and are in very different IPs.
Something is not right inside of Shield. Maybe in the last update, a bug crept into the code. I have never had an issue like this with Shield and now it is hitting all our accounts.
Only thing that worked for me was to log into cpanel > file manager and delete the entire plugin…
After that, I was able to get into the website… I then added a new plugin (Shield latest version) and was locked out again instantly.
So for now, we are NOT running Sheild at all because even you are logged out and got to the Home Page of the site, all you get is the big Shield warning and are locked out from even viewing the site.