The security error only shows that you attempted to enter a password in Security that has incorrect characters or was too short and this does not denote that a password has already been set.
Important - delete any existing XAMPP MySQL phpMyAdmin cookies in your browser.
Go to the Security page again and enter a password with more than 3 characters and letters and numbers that do not contain special characters, then add it again in the second box and then click on Password changing button which is a poor translation for Submit password, it does not mean you are changing an already existing password.
In the XAMPP Control Panel Start
MySQL but don't
tick the Svc check box and make sure MySQL is showing green running indication.
Next go to http://localhost/phpmyadmin
and enter root
in the Username box and then enter the password you just created then click on Go.
You should now be in phpMyAdmin with no errors.
Let me know how it goes please.
Good luck and best wishes.