Connect another working ext at the system end to the house cabling, so a differnt ext port is now using the same path to the jack. Use the same line cord as with x10. If the issue duplicates, replace line cord, if still, replace jack, if still switch to different pairs on house cabling, if still, replace whip from acs system ext port to house cabling.
Trying to plug in at the system port direct with a line cord was mentioned above so I skipped that. If it does not work at the system plugged in direct, bad port.
Other programming issues were mentioned above, check those first.
You do not always get what you pay for, but you never get what you do not pay for.