Siemens TK60001gb coffee machine shows 'no coffee' in error?

Sometimes the machine grinds the coffee (hopper is full of beans), starts the pump and then shows the little scoop (no coffee) and fails to brew the cup of coffee. On retry it normally succeeds.
I have removed the brewing unit and cleaned the insides thoroughly, but the problem persists. It seems like it might be a blockage in the funnel from the grinder to the brew unit, but I can't get to the funnel as it has a flap that is closed. I suspect this as after such a failure the next cup of coffee brewed is very strong.