Alright. My problem is solved now. and it was quite simple and makes a lot of sense.
When you synchronize Garnet VM for the first time, the software assumes an ID, so the device and hotsync manager will have the same ID. Which means that, if you try to synchronize your N810 with any other computer, it won´t let you, since the ID is not the same (and won´t tell you a damn thing about it).
With this information, I started to think about how to solve it.
In my case, I have synchronized the N810 with my PC for the first (I was testing and developing a DB for a client of mine). I have finished it, and now my client need to sync it on his computer, and as I stated before it haven´t worked.
Since I was not going to sync his N810 on my computer anymore, I did the simpliest thing ever: I have reinstalled Garnet VM and made the first sync with my client´s computer for the first time, so new ID´s were created. Voi-la! It worked. Whatta relief!
So, if your sync worked before with another computer and it´s not working anymore, just backup your Garnet VM and reinstall it. The end

Once again, tks again for all of your help, specially Addi!

So long,
Demian