It was a while ago now, so there may have been a conductive rubber sheet. Anyway, the only areas that would need cleaning would be the "dome" areas and the back of the rubber sheet.
As you say, probably the rubber sheet is worn at that point, so there is probably not a lot you can do,
These photos show this insides.
No amoujt of cleaning could sort out the Hit and Miss reliability of two other buttons. Fitting shims made from paper or masking tape didn't help either. Now electrical waste as we have a new one that works brilliantly.
This remote looks the same as mine. My 'Guide' button stopped working reliably, but I was able to repair it.
The keypad mechanism is different to most of the remotes that I have seen previously as there is another clear plastic film layer between the rubber button and the pcb track. The clear plastic film layer houses silver domes in the button positions. (This can be seen clearly in the previous photos). So the rubber button does not have a conductive pad that touches a pcb track when pressed. Instead, the rubber button makes a (physical) contact with a silver dome on the clear plastic film layer, and that silver dome then makes contact with the pcb track.
The clear plastic film layer is stuck to the pcb with adhesive, so I had to ease a thin plastic blade with kitchen roll soaked in alcohol based cleaner between the pcb and the clear plastic film layer to clean both the pcb track and silver dome that is operated when the 'Guide' button is pressed. Then gently re-stuck the film layer to the pcb.
Removing the clear plastic film layer completely would be too drastic; as it would be difficult to replace it accurately.