Can anyone tell me more about what this error means?
Devices Error (Not Readable): NotReadableError: Could not start video source
This error occurs when another application is already using a camera or microphone. I’ve seen this happen to Windows users when they leave an app like Zoom or Teams open. Tell them to make sure to close other applications (sometimes they don’t realize that they’re even open)
There can be several hardware or driver level issues, but the most likely reason is @Tasha's suggestion: Another application is using the device and ergo is not accessible to your application, this locking of a device or resource to an application is quite common to Windows platform.
In addition, this is also possible on Firefox, when several tabs are trying to capture audio simultaneously.