DIAGNOSIS Residual descaler solution remains in the internal water circuits. The standard rinse cycles may not have been sufficient to flush all lines, particularly the brew group circuit.
FIX Execute a manual, thorough flush of all circuits. 1) With the water tank filled with fresh water, run at least five cycles of hot water from the steam/hot water spout. 2) Run at least three full cycles of espresso using the 'Coffee' button without coffee in the grinder (bypass). Discard all output.
IF THAT DIDN'T WORK Descaler may have loosened scale that is now caught in the brew group or a final filter. Remove and rinse the brew group under warm water. Check and clean the small water screen on the brew group with a brush.
HEADS UP Never use vinegar or citric acid to descale this machine. It can damage internal seals and is nearly impossible to fully rinse out, causing persistent off-tastes.