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Let's make sure that the problem is indeed caused by Admin Tools and by which of its features. In order to do so, try the following:

Try to replicate the issue after disabling the "System - Admin Tools" plugin. If you can still replicate the issue (I couldn't understand if this is the case reading your original reply), it is not caused by Admin Tools. Disabling that plugin means that Admin Tools code (including the Web Application Firewall) is not running on your site.

If you suspect an issue with the .htaccess file, replace its contents with the contents of the stock htaccess.txt file shipped with every version of Joomla!. If you are on GoDaddy please wait for 1-30 minutes for the changes to be effective. Then, retry loading the problem page. If you can still reproduce the error, then it is not caused by .htaccess Maker and we can forget about the whole .htaccess aspect.

If doing any of the above did stop the issue from occurring, we'll have to do some troubleshooting.

First go to Admin Tools, Web Application Firewall, Configure WAF. Make sure "Log security exceptions" is set to Yes; if it's not, set it to Yes and click on Save. Now try reproducing your issue. Immediately after that, please go to Admin Tools, Web Application Firewall, Security Exceptions Log and go to the last page. The last log entry should have the date and time of when the issue occurred. Please copy the Reason and Target URL here so that I can help you.

If, however, you do not see a log entry, or the Date and/or IP address do not match your last access, this problem is not caused by Admin Tools' WAF. In this case, you will have to do some .htaccess troubleshooting. You may need to read the general .htaccess troubleshooting page, as well as the page on finding out necessary .htaccess exceptions.