Nespresso CitiZ coffee machine, some water is leaking into collection

Probably the brewing unit needs to be replaced. Here is the guide from iFixit (in German). The spare part costs about 25€.

Personally, I have never encountered damaged o-rings.

When leakage occurs, it is mostly because the intended outlet path of water/coffee is blocked:

Either the holes in the pyramid plate are clogged, or the actual coffee outlet port is clogged with dried coffee remains.

The water then simply seeks another way out ;-)

The dried coffee remains can also lead to bad sealing contact areas where the rim of the coffee pod is pressed against.

Only recently, I have encountered several Citiz machines, where the water leakage came from a (hairline) crack at the topside of the brewing unit.

To be more precise, the crack was on the top of the part which has the three teeth in it. It is barely visible to the naked eye...

Firstly, kudos to you for taking the initiative to fix the leakage issue yourself. It's always great to try troubleshooting before seeking professional help. However, I couldn't find the specific guide you mentioned. Nevertheless, here are some general steps you can follow to address this problem:

1. Double-check the water tank.
2. Inspect the seals.
3. Descaling the machine.
4. Check the drip tray.
5. Run a water-only cycle.

Better late then never I guess, and for all future references.

The issue is most likely one of 2 things. Gunk buildup at the 'filter' causing water to overflow into the collection box. This is easy to fix just by using a toothbrush on the inside of the capsule holder, towards where the coffee comes out.

If that doesn't help it's most likely gunk buildup or defective o-ring. You can get a replacement o-ring, i believe it's 33mm inner and 35mm outer. But a common problem with these machines is just a bit of gunk buildup. The o-ring is also quite loose.

To get to the o-ring unplug the machine, remove the water reservoir, and all other easily removable trays.

If you close the capsule holder halfway you'll see a clip used to hold the actual holder in place. The same clip is on the bottom. Undo both clips, twist the holder and it'll fall out of the bottom. There you can remove and clean the o-ring. No need to disassemble.

I've heard of some using pressurized air to blast it clean as well. But didn't have effect on mine.

Fair warining though, it's a real pita to get there. Took me a little over an hour to unlock both clips.