From page 22 of the Instruction Book:
Note: As a safety mechanism the espresso
machine is programmed to automatically stop
the water flow after 240mls, therefore you
will need to press the ‘MANUAL’ button again
to restart the water flow.
I reckon the 240ml is actually calculated and the function actually works on a timer.
My best guess is that the timer chip might be starting to misbehave.