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This is a fairly widespread issue. As you noted, you can restart to “fix” it. Another way to deal with it is to force quit. Here are step by step instructions from osxdaily.com:
2. Open Terminal, found in the /Applications/Utilities directory in OS X
3. Enter the following command string exactly, then hit return:
sudo killall VDCAssistant
4. Enter the administrator password when requested, this is required to execute a command with superuser privileges as prefixed by sudo
5. Relaunch the app that was attempting to use the camera.