When you create more than one site, each site needs its own port, the place where it listens for user connections and commands. The port for a site is often described as the data channel.
When you are running multiple sites, check your site settings to insure that each site has its own port. When two sites are both assigned the same port, you may have a port conflict. Only one site will be able to run at a time.
FTP sites
The default port for FTP sites is 21, but you can select any number between 1 and 65,535.
Note: Assigning a port number under 1024 (other than 21) may lead to conflicts with other programs running on your computer.
Assigning a port number under 1024 (other than the defaults) may lead to conflicts with other programs running on your computer.