DIAGNOSIS: The machine is brewing too hot. The Stilosa doesn't have a temperature control, so this is usually caused by scale buildup insulating the thermoblock, making it overheat, or just running it back-to-back without a cooling flush.
FIX: Run a descaling cycle with a proper espresso machine descaler (like Durgol or De'Longhi's own brand). More immediately, before you pull a shot, run a blank shot (water only) through the brew head into a cup. This flushes out super-heated water from the system. Then immediately lock in your portafilter and pull your shot.
IF THAT DIDN'T WORK: Your grind might be too fine, causing over-extraction (which tastes burnt). Try a slightly coarser grind. This machine works best with a pressurized basket, so it doesn't need an ultra-fine espresso grind.
HEADS UP: Never use vinegar to descale this machine. It can damage the internal aluminum components. Use a citric acid or sulfamic acid-based descaler made for espresso machines.