DIAGNOSIS The mechanical linkage for brew stop switch (the small lever on the brew basket arm) is worn or the internal spring mechanism has failed.
FIX With the machine off and cool, inspect the small black lever on the side of the brew basket holder. Move it back and forth. It should click positively into the 'stop' (forward) position and stay there. Try engaging it without the carafe present to rule out misalignment with the carafe lid.
IF THAT DIDN'T WORK The issue is internal to the brew basket holder assembly. The spring or plastic catch inside is likely broken. The entire brew basket holder unit must be replaced.
HEADS UP Do not force the lever. Using excessive pressure can snap the plastic. If the switch won't engage, you cannot use the 'pause and serve' function. Brew a full carafe instead.