Brake Fluid Leak Caddy 2K - Rear Left

Hello.
I bought a Caddy 2K two years ago and have been working on it on and off ever since.
Now I was planning to go to the TÜV (German technical inspection), but the brakes on the left rear wheel aren't working at all. The brake caliper moves freely, and the handbrake mechanism is also working.
Fluid is present on all brake calipers except for the rear left one. I then worked my way through all the fittings, all the way to the ABS module, but nothing is leaking there either.
However, if I select the "bleed hydraulic unit" option in the basic settings via VCDS, brake fluid also reaches the rear left brake.
Since I've owned the car, only the right rear saddle has been replaced. Otherwise, no work has been done on the braking system.
It's a 2004 Caddy with the MK70 ABS system.

Do you suspect the problem is more likely related to the control unit or the hydraulic block?

Thank you very much for your ideas.
Sören.