Ugh ... I had that same issue with my old Fluval 404. I'm assuming Eheim canisters seal similar to Fluvals so check (assuming you haven't already) the O-ring to ensure it's in good shape ... then ensure the clamps are on tight enough. This was the problem with my 404 in terms of leaking. Towards the end I'd have to literally push down with my body weight to get it to seal.
I'm certain someone who has that same filter will chime in soon enough.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.