It means what it says – bad checksum in internal ram.
You can fix this with "fix internal" on the <setup> <service> menu. Look around p2-23 in the UM for more info.
If it comes back, this will be a sign that the internal battery is fading. These last about 10 years, so the time could be right. If it is, you can buy one from the Eventide Store. The battery is built into the memory chip, so they are not cheap.