I got this error when I was running my xampp on my ubuntu
redha@redha:~$ sudo /opt/lampp/lampp start
[sudo] password for redha:
Starting XAMPP for Linux 7.3.27-1...
XAMPP: Starting Apache...fail.
XAMPP: Another web server is already running.
XAMPP: Starting MySQL...ok.
Had the same problem. (This answer) solved my problem. For the purpose of this question, I shall re-iterate it.
Stop all services
sudo /etc/init.d/apache2 stop
sudo /etc/init.d/mysql stop
sudo /etc/init.d/proftpd stop
Then restart the server
sudo /opt/lampp/lampp restart
