The config.inc.php should be readable.
Give it 755 permission and try again.
chmod 755 /opt/lampp/phpmyadmin/config.inc.php
Well actually 755 is little wide open.
Try 644, it is more secure than 755
chmod 644 /opt/lampp/phpmyadmin/config.inc.php
But it does not work in my phpmyadmin that's why I use 755, it works for me.
If 644 works for you then excellent.