Thanks for your reply!
Well, I'll explain in more details then. Actually, I didn't get any errors. I had "index.php" as a part of my site's URL. And it didn't look too nice to my taste. To get rid of it, I had to enable URL Rewriting in a back-end of Joomla. And to do that I also had to RENAME Joomla's htaccess.txt file in root to .htaccess (I run Apache). And I couldn't have done it since I can't have two files with the same name in one directory. I was afraid to just delete Admin Tool's .htaccess file, so I decided to completely remove Admin Tools and then re-install it AFTER I already had Joomla's htaccess.txt file renamed into .htaccess And what happened in that case was that Admin Tools AGAIN created its own .htaccess file and Joomla's original .htaccess file it renamed into .htaccess.admintools or something like that. I then compared .htaccess.admintools to the Joomla's original htaccess.txt file (I took it from a new Joomla installation archive I downloaded for that purpose only) and found both files exactly identical. Meaning that Admin Tools didn't change the content of Joomla's .htaccess file, just renamed it. The last thing I did was deleting Admin Tools's .htaccess file and after that I renamed .htaccess.admintools into .htaccess That's all!