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Have you tried installing Admin Tools Professional again, twice in a row, without uninstalling it before or in between?

Thank you for checking. Unfortunately, this confirms my suspicion: your host is blocking access to this page. Most likely they have set up security on their web server (e.g. mod_security2 on Apache) with a rule which blocks access to this page. It is very likely that it's a rule which blocks access to the page when a URL parameter contains the term htaccess. Note down the exact date and time you try to access this page and get a 404, as well as your IP address. You will have to ask the host to go to the server's error log for that exact timestamp, URL, and IP so they can see why it's being blocked and help you address it. Most usually they can add a simple rule to your existing .htaccess file to disable the rule which blocks access to that page.