When I checked from below the vehicle, an already familiar failure was obvious:
This time, I wasn't interested in yet another cylinder repair, I ordered a new one. While I was at it I noticed that one of the contact points on the starter motor was broken, too. Somebody messed with it already, as there some traces of silicone and bodged repairs...
A new starter costs ~300 € - but the starter itself was working perfect, only the starter magnet was bad. Luckily I found a new replacement starter magnet for 30 €. The repair was a little tricky because someone has been here before:
Now on to the clutch cylinder story: The repair set from Porsche was definitely o.k. - I guess it was me who messed up. The inner surface of the old cylinder just wasn't clean and smooth enough, before I re-assembled the service kit. It only took 6 month to wear out the new seal. This time I ordered a brand new cylinder:
Here it is, already assembled to the clutch bell housing.