You should check the following:
* Verify that the MacX25 Admin program is running, and has started the MacX25 server assigned to the port's serial card.
* If necessary, start the MacX25 Admin and server programs, and then try again to turn on the port
* If the Apple Remote Access MultiPort Server starts up automatically from the Startup Items folder, verify that the MacX25 program is also set for automatic startup, and that it starts up before the MultiPort Server program. In addition, make sure the at you have selected the "Auto Start" option for the MacX25 server in MacX25 Admin's Server Info dialog box.
* If MacX25 is running, and you still get errors when turning on X.25 ports, then check the server log for additional information. If the log indicates that the port's serial card is in use by the Modem Service, use the Remote Access Manager to turn off the modem ports, and then try to turn on the X.25 ports again. You can't have modem ports and X.25 ports active on the same serial card.