I'm using macOS 10.14 Mojave. If I try and send a transaction from a Parity Signer account, after I enter a recipient address, an amount, and click "Scan", and then scan the QR code using Parity Signer and click "Sign Transaction" on my mobile device, then when I click "Next Step" in Fether it displays "Webcam hardware error. Try restarting your computer", which corresponds to a NotReadableError occuring in fether-react/src/Scanner/Scanner.js. Restarting the computer does not resolve the issue. I've debugged the error in the catch block to be DOMException: Could not start video source.

Note that running sudo killall VDCAssistant; sudo killall AppleCameraAssistant does not resolve the issue.

Note that I have all the latest Software Updates installed.

However, if I go to "Finder > Applications > PhotoBooth", it opens and the FaceTime HD Camera (built-in) webcam starts working immediately with the green light illuminated on the screen.

The solution will need to update the error message that corresponds to NotReadableError, since restarting the computer may not be enough.