Acrobat X Pro always ask licence number

I have a perpetual license to the CS6 suite, but
I only use acrobat X pro.
Lately the program always asks me for the license number. I fill it in in the appropriate space, but the next time I open a pdf file the program asks me for the license again.
I have already uninstalled the entire suite and reinstalled only acrobat, I tried to download the suite from the portal corresponding to the license in use.
It doesn't work.