Roland RD-88 "Panel not respond" error

I bought this second hand.

One of the first times I powered it on, the screen displayed “panel not respond”, and it had a clock timer counting upwards at the bottom of the screen.

None of the keyboard button lights were on, and the keyboard didn’t work at all.

I powered it off, and since then, it has worked fine each time.

I have 30 days to send this back, as I bought it using paypal goods and services.

I’m worried I’ve spent a lot of money for a keyboard which might have an intermittent and potentially expensive fault.

Does anyone know what causes this error, and does anyone have an Rd-88 service manual?