Hey jod_apple,
Thank you for your suggestion. I tried to reset NVRAM. But the result is the same.
I also tried to reset SMC with the same result - unfortunately.
After each start of the computer, I can see (in the system report) that the Mac mini has found the camera and speakers in the Thunderbolt Display (It is also displayed in the sound icon under the system setting). But once I start FaceTime, the green light next to the camera in the display blinks one time, and then FaceTime reports that no camera is available.
So is there another way of getting the Mac mini to work as it used to do (a few days ago) ?
I tried to connect the Thunderbolt Display to my MacBook Pro (using the same cable as used with the Mac mini). Now everything works fine (built-in camera, speakers and microphone). The MacBook Pro (15-inch, 2017) runs the macOS Big Sur version 11.2.3.
So the issue must lie in either the Mac mini or the latest version for the macOS (I use version 11.2.3 - macOS Big Sur) - or both combined.
What could I try to solve the problem ?
Friendly greetings
Per