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erkal - is the gasket the right way up? - if it is (flat side, not bevelled) facing down, than the only thing I’d suggest is that your gasket has been worn out prematurely, or is damaged. Saying that, you said it doesn’t happen with the spouted portafilter, correct? - which then rules out the gasket theory, and the only possible explanation is that there must be something wrong with your portafilter, which I never heard of. Very strange.