Taking the whole tank apart I realized that one of the plastic ties that helps pushing the rubber sealant rings against the bottom of the tank is cracked (apparently I turned the screw a bit too tight).
OK, no worries. There is a set for sale for this… only the sealant rings and the screws.
I went to the shop, bought the set, and took everything apart. Again.
Apparently the screws coming with the new set are way too short… and since they are thicker than the original I can’t just switch the screws using the original sealing rings.
To be continued.