Diagnosis
The black rubber O-ring seal at the top of the filter housing is either missing, damaged, pinched, or has debris on it. That's the only thing preventing a leak at that seam.
How to Fix It
Turn the bypass lever to 'off' or shut off the main water supply. Unscrew the housing again. Inspect the O-ring seated in the groove at the top of the housing. Clean it, ensure it's not twisted, and lubricate it lightly with a dab of silicone grease (or just water) before re-seating it. Hand-tighten the housing firmly—no wrenches needed.
If That Didn't Work
The O-ring might be nicked or flattened. The replacement filter pack should include a new one. Swap it out. If it's still leaking, the plastic housing threads or the cap itself might be cracked from overtightening.
Heads Up
Never use tools to tighten the clear housing. 'Hand-tight plus a little extra' is the rule. Overtightening distorts the O-ring and can crack the plastic, causing the leak you're trying to prevent.