AC450 Controller Issue
Is this controller at T****t , Australia ? If so, all the CPU firmware was upgraded about 4 or 5 years ago to be compatible with 800xA, so it should be newer than the original 1997 delivery. If this is the same site, I think the SC520's have been in use for several years.
I assume you completely cold started the controller as part of troubleshooting ? On the other node with this problem, what did they actually do to be able to "go back to using SC520's" ?