Is the ACmV reading stable?
What about different (lower) frequencies. What about linearity - how is it at 25, 50, 75mV; still -5.16% ?
We don't have a service manual or schematic to troubleshoot the ACmV path, and it's a custom DMM IC. Right to repair nah.
The relays are kinda cheap so try working them, I don't think they have a lot of palladium lol. But I think the cmos muxes are doing the range switching work. If it was grossly overloaded on ACmV the H/W could be damaged, that's why I suggest seeing if it's just a span problem or more.
There are two firmware loads - CPU for the main and MCU for the DMM portion and not sure if any F/W update covers both processors.