Lambda Sensor Error

Hello!
I own an Audi 80 (B4) built in 1993 with a 2-liter gasoline engine (Engine Code: ABK (115 horsepower Digifant)). Mileage: 172,000 km.

I was now able to pinpoint the problem:
If I drive 10 km on country roads after a cold start, the engine runs perfectly (water temperature around 50°C). However, if I drive in city traffic, where the engine heats up quickly, the power suddenly disappears. The RPM remains approximately the same. Without load, I can rev the engine up. When I engage the clutch again, the same thing happens. The engine doesn't stutter or run unevenly - the idle is also fine; it's just that there's no more power (it feels like the car is being braked).

I had the fault memory read out at VAG:
00525 028
Lambda Probe G39
Short circuit after plus

Does anyone recognize this error message?
How can I check the connection lines or where do they run in this vehicle?
and how do I change the L-probe on this vehicle?
Thank you for your help,
Best regards, Patrick