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That redirection is (obviously) wrong as I'm sure you noticed yourself. It doesn't have a domain name! It also doesn't come from Admin Tools.

My first troubleshooting tip would have been to check your .htaccess. You said you tried a file from a working site. Have you tried using either the contents of Joomla's htaccess.txt or just deleting that file completely?

If that doesn't help, try using a different browser. If the problem was something temporary it's also likely that your browser "remembers" the wrong redirection even if it's no longer occurring. This browser behaviour can set you down a path of hunting windmills very easily – the first time it happened to me, many years ago, I spent about two hours being increasingly frustrated as to why the code I had in front of me seemed to be ignored during Joomla execution, only to figure out that it was not actually executing, the browser had just cached the redirection and never hit the server.

If that doesn't help, check your configuration.php file. If your $live_site is not blank please try making it blank, i.e. so that its line reads:

public $live_site = '';

If this still doesn't help my suspicion would be a third party system plugin, especially anything that has to do with SEO, SEF or site optimisation.

I am glad I could help you out :) Have a great rest of your weekend!