Thanks everyone for answering my question.
I've found the issue.
The problem was that I tried to access specifying the tfs like this
tfs.site.com:8080/tfs/collection
but it was wrong. Been at server I just had to specify the url like this:
tfs:8080/tfs and that's it.
I can now access correctly to my tfs from server.
Again thank you everybody for our time.
Cheers.