The problem was that I'm under a Novell Network, if I use the automatic mapped drives, the push fails.
This post gave me the clue: http://git.661346.n2.nabble.com/Problem-pushing-to-a-Novell-share-td7248875.html
The problem seems to be that Novell Networks throws a Error 80 "File exists" if a directory does not exist, while it actually should be Error 3 "Path not found". It all happends in create_tmpfile() function in sha1_file.c
And here it's been one more time discussed Git push fail to a Windows share
So the workaround is to use the ip address of the mapped drive (as remote repo) instead of using its UNC path or the assigned drive letter.