Precision 7540, P3421W, no USB-C signal

Hi - I am using a Dell Precision 7540 and in my office I connect the notebook to a Dell P3421W via USB-C.
This worked well for over a year and today it stopped working?
I arrived in my office and the monitor wrote: "no usb c signal from your device".
No problems at home with a dockingstation and two monitors connected.

I got recently (03/09/2024)  two Updates
- BIOS 1.3.40
- NVIDIA 31.0.15.5274

Any ideas?