I had a look at some of those documents. In this one...
https://www.cmco.com/globalassets/inriver/resources/cs00407-rv12.pdf...on PDF page 98, there is a circuit board layout diagram. A callout for U50 is there on the right side of the print and it points to U56 on the circuit board (not sure why the discrepancy). Anyway, U56 is the NV-RAM and the docs say to repalce every 8 years and it is rated for 10 years.
That sound very much like a device that was popular in that time frame from a company called Dallas Semiconductor. They had NV-RAMs that had embedded, non-replaceable, lithium batteries.
Depending on which one it is, there may be a replacement. I've also seen YouTube videos of folks hacking them apart and putting an external battery on it.
This could be your problem as well.
Would be good to know exactly the sequence of events and the error you are getting.
