In your issue title you say this happened after upgrading Joomla itself. In your pst content you say this happens after updating Admin Tools. This inconsistency makes it really hard to help you. Moreover, you don't have a version number for Admin Tools or tell us which version you updated from which makes it even harder for us to help you.
So let's start by completely disabling Admin Tools.
Delete the following files and folders (not all files and folders may be present):
- .htaccess
- administrator/.htaccess
- administrator/.htpasswd
- plugins/system/admintools
- plugins/actionlog/admintools
- plugins/installer/admintools
By doing that you have removed everything that Admin Tools has done and all of its code which can run on your site.
If this didn't help your problem is unrelated to Admin Tools.
If that worked you probably had a partial installation of Admin Tools. Download the latest version of Admin Tools from our site and use Extensions, Manage, Upload & Install in the administrator backend of your site to install it twice in a row, without uninstalling it before or in between. Afterwards go to Admin Tools, .htaccess Maker and click on Save & Create .htaccess.
If the last step caused your problem to re-appear then it's some settings in the .htaccess Maker that you enabled which prevent access to your site. Delete the .htaccess file from your site's root to regain access to your site and follow the documentation instructions, see https://www.akeeba.com/documentation/admin-tools/athtaccess500.html
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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