Error code.

[Rufus McDufus]

Mains voltage can vary quite considerably over the day.  I live close to a substation and usually get up to 253V at certain times (which is just on the cusp of the maximum for the UK) but it can drop to 245V at other times. Might be worth measuring it somehow. There are cheap plugin monitors available (power monitors will often display the input voltage). A few years ago I borrowed a data logger from a local hifi dealer which measures mains over time.

However if just one unit is reporting overvoltage then that's a bit odd.