There are several possible causes of this fault :
1- The coffee is too finely ground: check the grinder settings.
2- The filter holder gasket is warn and must be replaced.
3- The filter holder is badly positioned on the brewhead.
4- The sieve is damaged.
5- The grinder's grinding wheels are damaged and must be changed by a technician.
6- The sieve's holes have become larger: change the sieves.
7- The pump pressure is too high (>10 Bar).

Case 1 can be resolved by adjusting the grinder settings. The other cases require the attention of a technician, who will carry out the necessary maintenance operations.