According to the information I have found it seems to be the tab key on the keyboard is sticking.
Instructions I found read: -
Power Off and remove PSU and battery.
carefully move the keyboard plate (curved piece of plastic directly in front of the pointer buttons)
You will see a small metal plate holding the keyboard in place unscrew this and tilt the keyboard upwards.
You will notice that the keyboard in plugged to the motherboard via a ribbon cable you will need to undo this (to do so very carefully push the left and right sides of the connector and you will notice it moves up releasing the ribbon cable. Plug in the power and power up if the system starts then the issue is with your keyboard and you may need to get a replacement (first ensure the TAB key is clean (remove and check under the white nipple).
To reassemble follow instruction backwards
Hope it works okay