So, I tried to uninstall AVAST then reinstall it and it worked...

However, it doesn't feel right to be forced to unistall/reinstall a software, especially an antivirus, to get rid of such a stupid bug...
I happen to be a computer animator in a local association and frankly, these kind of small bugs for computer profanes/beginners are the perfect thing for them to quit using a program. And I feel dumb to them, cause they have installed AVAST on their own computers following my advice and they are experiencing the same annoying tab bug... I consider myself to be an advanced computer user and these kind of bug are already enough to deeply annoy me.... Imagine with casual users, who are full of bias and prejudices with computers, you don't have a clue about how something, no matter how small it is, can drive them crazy cos' "they expect the machine to know better".... You know... And somehow, they are right ! You can understand major bugs, shit happens sometimes... But small things like this, no... It feels like the little rock in you shoe, it's completly dumb and unjustified...
Anyway, I hope my experience can help people around.... And help AVAST developpers to avoid these things in the future...
Regards,
Jonathan