Diagnosis
The filter basket is clogged or the water flow is restricted, causing pressure to build and "burp" back through the grounds. This is almost always a cleaning issue.
How to Fix It
Descale the entire machine and deep-clean the spray arm. Run a full brew cycle with only a carafe of white vinegar, then run two cycles with clean water to rinse. Remove the shower head/spray arm (it twists off) and soak it in vinegar to clear any mineral deposits in the tiny holes.
If That Didn't Work
Check the silicone seal (orange gasket) on the filter basket holder. If it's warped, cracked, or missing, it won't seal against the basket, allowing grounds to bypass. Replacements are available.
Heads Up
Never use abrasive cleaners or a toothpick on the spray arm holes—you can warp them. Use a soft brush and vinegar soak only.