Diagnosis
The air vent hole(s) in the upper chamber are blocked. The system needs air to escape the lower chamber for water to flow down through the filters. No vent = hydraulic lock.
How to Fix It
Locate the small vent hole near the top rim of the upper chamber. It's a tiny drilled hole. Use a toothpick, paperclip, or pin to gently clear any debris, mineral buildup, or even a stray insect that's clogging it. Ensure it's completely open.
If That Didn't Work
If the vent is clear but it still overflows, the filters might be so severely clogged (see the slow flow problem) that they can't pass water fast enough even with proper venting. You need to scrub or potentially replace the filters.
Heads Up
Never seal or cover that vent hole thinking it's a flaw—it's a critical design feature. Also, make sure the upper chamber is sitting level on the lower chamber; if it's tilted, water can find a high spot to spill over.