I had the same problem. Also when the grill is off, it works again. Its a design flaw. Problably a case of creep of the plastic cover.
The cause that the buttons are not working is that the button below the power button is constantly pressed when the grill is on. I found out that it also works when you very slightly lift op the plastic around the jammed button by putting your nail inbetween the plastic and the grill.
When this doesn't work you can try to just slightly release the six screws under the grill.
Combining this solutions I got it working again, with the grill on.