Reboot the GC.
Check the terminations with a $20 HD/Lowes cable tester.
Use a different port on the EZport. Similar to what jautor said, bring the keypad to the EZport and try a different port.
Retrieve the configuration program from the GC, make a minor change, and send it back.
Any remodeling done recently? Nailed/screwed cable as the cause? New kink in the category cable?
Sometimes waiting a day will fix the issue, magically.
RS485 networks can be finicky. How many total zones, and what size house? I think there is a 3000 foot limit total to the RS485 network, with each category cable length doubled (to keypad and back).
Unplug every keypad, and add them back one at a time, confirming each keypad works as it is added.