It could be that the center wicking tube has a hole in it (possibly a burn-thru hole) or has come undone at the bottom where it seals to the bottom of the unit.
If it is not damaged in the above mentioned manor, then a cleaning might fix it. Soak it for 15-20 minutes in near-boiling water and then rinse thoroughly using the "Turkey Baster" method. Spin dry or air-dry over night, and fill about 3/4. Check for continued leaking. If it still leaks, it's a goner....
Hope this helps,
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