macOS 10.13: FaceTime preferences greyed out I started Facetime and went to preferences to Signout from my old AppleID. After successfull signout I wanted to Login with the new one, but Facetime preferences are greyed out.

I'm on macOS High Sierra (10.13). Any ideas on how to fix this and get back to Facetime again?
Found the issue by accident. As I got an external monitor attached Facetime couldn't connect with the integrated Webcam of my Macbook. Therefore it didn't show up the login screen.

As I opened the Macbook the Webcam started working and I got the required Loginscreen to get back to Facetime again.