I had the same problem when I replaced the filter on my 560SEL.
I ended up changing filter brands with no success. I used a different washer with no success either. I ended up tightening as best possible then applying some sealer as Ferdman directed and let dry. So far so good

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6