Face ID suddenly stopped working (iPhone 15 Pro, iOS 18.3.1), can't setup a new one

Two days ago, all of a sudden, when I was about to take a picture with the rear camera, the camera screen went black. I restarted the iPhone, and then the camera worked fine. However, after this, Face ID stopped working, and I had to enter my passcode every time instead.

Both front and rear camera work fine.

I removed my existing Face ID, and when I attempted to create a new one, I received the following error: "Face ID Is Not Available. Try setting up Face ID later." A square window appears at the top of the screen with the front camera turned on (see attached screenshot). After clicking 'OK,' the setup function closes, and nothing happens.

I tried the following steps to set up a new Face ID profile:

1. Restarted the device >> Didn't work
2. Reset All Settings >> Didn't work
3. Checked the front camera >> No visible dirt or damage
4. Turned off "Attention Aware Features" >> Didn't work (and is not particularly relevant, as there is no active Face ID since I deleted the original one)
5. Checked "Content & Privacy Restrictions" (turned off, as suggested by someone) >> Didn't work