https://koenigworld.com/d...I had exactly the same problem, and I bought a new o-ring from the link above. Actually I bought two (one for next time) and a bottle of descaler, just to make the postage worthwhile.
Getting the cover off is the hardest bit. Use a torx screwdriver (size T8x50) to remove the 4 screws at the bacl of the machine (having disconnected the electricity supply). The cover comes off backwards. Move it gently but firmly to the back, about 6mm, and this will free its fixings from the body of the machine. Then pull the sides outwards, and just lift it away.
The o-ring you need to replace is around a piston which is above and to the right of the brew unit. You may find the old one has gone, and you just have a slot around the piston to put your new one into.
Lubricate the ring with some silicone grease (available on Galaxus), and use the end of a teaspoon to ease the new o-ring into the slot. It takes some care as the edge of the piston is close to another component, but with enough lube, and a tool that is firm but not sharp, it goes in ok.
Before you put it all back together, clean the interior, and grease the moving parts with the silicone.
Reassembly is the reverse of the above. If you've done it right, the drip tray will be dry (after a few cycles) when you make a cup of coffee.
Total cost of fix: CHF25.
Hope this helps. Don't work on the machine with it connected to the electricity! Empty the water and coffee out before turning the machine over and before removing the cover!