Usually leaks in that location mean:

1. The filter was re-tightened unevenly. On many large clamps like that, you need to tap them with a rubber hammer as you tighten, so the clamp can slide and tighten evenly.
2. The O-ring has deteriorated.
3. The groove the O-ring was in OR the O-ring itself was not rinsed 100% clean before re-assembly.

Waterbear can probably think of some other reasons.