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Yes, it's your browser.

I know it sounds stupid, and I sure as heck thought I was hallucinating when I first discovered this problem a few years ago, but I promise it's true.

You know how your browser shows thumbnails of the most recently visited sites when you open a new tab / window? Now think about when and how these thumbnails are generated. Yes, it happens in the background while your browser is open, or when you open a new browser tab / window. The browser visits that URL, and if the result is HTTP 200 OK it takes a screenshot of the page it loaded in the background, makes a cropped thumbnail out of it, and shows it to you.

So, yes, you are not visiting the site, but the browser is.

It's bonkers, isn't it?!

Luckily, there is a very simple workaround to that problem in recent versions of Admin Tools.

Go to administrator, Components, Admin Tools for Joomla!, Web Application Firewall, Configure WAF. Click on the Basic Features tab. Set the “Browser cookie override for the administrator secret URL parameter” to Enabled. Click on Save & Close. Now log out, and log back into your site. This creates a cookie in your browser which securely tells Admin Tools that this is a browser which has previously used the correct Administrator Secret URL Parameter to view the backend of the site, bypassing the check that got you blocked in the first place.

Ah, yes, that explains why it had not worked the first place. The same thing happens in the (equally rare) condition that you've set up your browser to remove all cookies after closing it down.