When you said,
"... I just Installed 4.1.2.0 and the The Key and it works Fine..."
Are you saying that you entered the Fab 5 key with the Fab 4 application?
I doubt this would work.
I suspect you may have inadvertantly retrieved the Fab 4 key again, if in fact, you just activated Fab 4 with a key.
On Fengtao's support page:
http://www.dvdfab.com/retrieveregkey.htm
There's a dropdown for key retrieval. The default key right now is
DVDFab 3.x/4.x.
Please click on the dropdown to display:
DVDFab 5.x Beta