My guess is that its a battery-backuped RAM where the battery has gone low, you probably have to change the NVRAM chip. They will only hold for about 10 years, even when only stored, and even if no current is drawn from the internal battery.
It could also be some other kind of NVmemory like Flash or EEROM,where one has written to many time to it, but if so the error code is missleading because its says NVRAM so it must be like above.
Try updating your RSLinx to the latest version.
I had a similar problem, and found the version of Linx I was running was not compatible with the version of the DHRIO card and was reporting errors incorrectly.
I got a similar error when trying to connect to the control logix through ethernet. Then I saw the note sent with the processor. New control logix processors do not come with any firmware. You need to flash the PLC then you can communicate. Hope this helps!
