SOLVED!!!.
First of all, sorry Jerome for the late reply.
Your comment about the "error message" was the key point to realize that on my site you have to scroll down to read the error message. So now I could read "you are not a vendor".
I checked all the vendors configuration and I added the administrator as a user too. And it works again. I knew it was something very simple, but it took me a long time. Sorry, as you can see, I am not a programmer.
Maybe I am wrong, but months ago (or even a year ago) I think that it was not mandatory to add a user too. You could just log into the Vendor Control Panel just with the administrator user.
Anyway, thanks a lot for the quick reply, support and such a great software. Amazing.