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There is no such message in Admin Tools. Are you using another third party security plugin or a web application firewall external to your site?

If the message is something you have entered in Admin Tools to display when blocking an IP address please let me know. My answer will be completely different in this case.

OK, that makes more sense and it's easy to understand.

You have enabled the Administrator Secret URL Parameter feature in the Web Application Firewall.

While you were working on your site's backend you stayed on the same page for an amount of time longer than what you have configured as the Session Lifetime in your site's Global Configuration. This means that your session expired. Therefore you are practically logged out at this point.

The very next thing you tried to do was to save something or navigate to another backend page. However, since your session expired you were logged out. Therefore you are trying to access an administrator page without being logged in and without providing the Administrator Secret URL Parameter. This causes a security exception in Admin Tools.

Moreover, your Admin Tools settings indicate that repeated security exceptions should result in the account being blocked. This happened and that's why you got blocked.

The correct solution is to increase your Session Lifetime to something more reasonable. On most of my sites I have it set to 30'. On my blog site it's even longer. This means my session will not expire when I am working in the backend of my site and I don't get blocked.