Thank you Patrick for your suggestions and considerations.
Instead of trying other apps I spent some time doing experiments with possible conflicting apps. I do not have specific technical data, but I can say now that KIK social app, which gives also access to the cam, seems to be conflicting with the normal functioning of the default cam of my FP2.
I state this because cam seems now to be normally working when KIK app si closed and FP2 is rebooted.
I would like to be able to say that when KIK app is open cam doesnt work, but it doesnt happen always.
At least I found a workaround 
Thank you again
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