The central spout dispensing unit is not correctly fitted or is wrongly assembled.
Remove and dismantle the parts of the central spout. Reassemble the parts in the right order and reinsert them.
Proceed as follows:
Remove the stainless steel cover to the front, see Fig. 1.
Pull the dispensing unit forwards to remove, see Fig. 2.
Twist the upper part with the holder for the milk pipework. Pull off the upper part, see (1) in Fig. 3.
Pull off the Y-piece as well, see (2) in Fig. 3.
Loosen the dispensers, see (3) in Fig. 3.
Reassemble the dispensing unit, see (4), (5) and (6) in Fig. 3.
Push the dispensers firmly back onto the dispensing unit.
Refit the dispensing unit into the central spout.
Refit the stainless steel cover.