Climate Control A4 Malfunction?

Hi.
I'm having a problem with my A4 TDI (year 1996) that has automatic climate control. When the car is parked for an extended period (3-4 hours), the air conditioning works perfectly fine (set to around 22°C) until the engine and interior warm up. Then, it only blows cold air into the cabin. There are no error codes stored, and I've already performed the basic reset procedure. Perhaps someone has another idea what could be causing this.
Best regards, Bolzer.

Does the fan only blow cold air when the compressor is running, or also when it's turned off?

Perhaps check the temperatures using measurement blocks to verify their plausibility.