1) Is the leaking water cold or warm?
-> This tells you whether the leakage originates upstream or downstream of the thermoblock.
If the water is warm/hot:
2) Does the water leakage only occur when operating with coffee pod, but not when operating without coffee pod (i.e. only water)?
And after it occured, has the main lever got very hot and wet and its underside?
-> In this case it could be a thin crack in the top of the brewing unit.