Diagnosis
The O-rings on the filter cartridges are not seated correctly, or the housing O-rings are pinched/dirty. Even a small grain of sand can cause a leak.
How to Fix It
Turn off the feed water valve and unplug the unit. Unscrew the leaking housing. Remove the filter. Inspect the black O-ring on the top of the filter. Wipe it and the groove clean. Lubricate it with a tiny amount of food-grade silicone grease. Re-seat the filter straight down. Hand-tighten the housing firmly.
If That Didn't Work
The housing's internal O-ring might be damaged. The housing has a large flat white O-ring inside the cap. Remove it, clean the groove, and inspect for nicks. Replace it if damaged (kit usually comes with filters).
Heads Up
Never use petroleum jelly on O-rings. It degrades the rubber. Only use silicone grease made for plumbing or the grease supplied with the system. Overtightening with a wrench can crack the housing.