No lights but beeping, HS 500

[BillP]

We're having an issue with HS 500 UPS. It has been randomly, but mostly late evening/night, going into a mode where all the lights are off, but giving a long beep every 5 seconds.

When it does this we can unplug all the load, and the mains input, and it keeps doing it. The only way to stop it is to turn the unit off, then on again.

One time the unit powered back up on it's own after 6 hours.

I've just had this happen again, and it's web portal says Over Voltage.
We have replaced the unit with a new HS 500 and had exactly the same issue.

Is this the expected, but un-documented behaviour for an Over Voltage situation?
Why doesn't the unit go back to normal once the Over Voltage has stopped?

I've modified the limit from 256v to 266v to see if it will stay on.
Dave