I've just re-installed FCamera and drivers it goes on OK!
Maybe it's because I've not rebooted the machine ... I'm doing it right right now, let's see what happens ... Oooh la la! It works! Definitively it's not a matter of FCamera and its drivers.

Maybe the reason was "abook-qrcode" application. I forgot to mention this app, but it was just one of the latest using camera to be removed from my phone before the solution. Now I'm afraid to reinstall it ... but here I go ... OK, re-installed and rebooted! It still works. was it a problem with "catblock" application and its multiple errors spread through the system?