Just a generic guide, on old electronics. My Minolta III-F meter has a hell of a time waking up after the battery has been out of it for a while. If I leave the battey in it, it is flat in 2 days. Obvioulsy this was one of the reasons there was a IV-F meter.
Try a new battery. Leave the battery in it for a day at least after putting it in, then see if error code clears.
If not, try leaving the meter with the battery out for a day before powering it again.
A lot of old meters use an electrolytic capacitor to keep the processor powered to keep factory calibration settings stored in some registers.
There may also be a long life lithium battery soldered to the pc board that does this task, that may have been designed to last 10 years.
That was the case with some moving lights I fixed at the theatre last week.
They intermittently would forget thier address after being powred off a while.
A lithium battery and electrolytic powered the cmos.
The electrolytic cap dried out and that left the lithium to work harder; it was rated for 10 years, but died after 7.
$17 in parts, and a $2000 to replace light is now relaible again; my time to know what parts to replace and how was free, since I volunteer for the group.