-50 error in Itunes

Good evening, since 2010 or 11, I am in possession of a book air. No issues until now.

MAC OS 10.6.8

But since few months I can't finish saving files of my Iphone or upload new version of Mac because it is sayed an unknown error has occured -50

Une erreur inconnue s'est produite (-50) as soon as I open my Itunes Appli...

I have tried to find solutions on forums but nothing really succeeded.

I thought it would be better if i would upgrade to yosemite version but now when I go to apple store to upgrade. As they ask my Apple ID + Password..aplle says in red that they cannot recognize my computer...

It is a computer I bought officially to a mac store in NY few years ago. I used to live there. Now i am in Paris. Might it be the problem?

thank you