i found out the problem. it was a poblem with my fileystem. the filesystem isn't able to craete unnamed tempfiles (tempfile.TemporaryFile()).
but this feature is used by the FieldStorage class in the cgi module.
i have solved the problem like that:
class MyFieldStorage(cgi.FieldStorage):
def make_file(self, binary=None):
return tempfile.NamedTemporaryFile()
then i have used the FieldStorage Class native and not throug web.py.
thanks anyway.