Robert Schmidt reported the problem here:
https://groups.google.com/forum/#!topic/ns-3-users/zMEMJ7vPORY

The bug is evident: if you use ConfigureDefaults before ConfigureAttributes the first will read the whole file, leaving the stream with the eof bit set.
Since seekg(0) doesn't clear the eof bit (in c++99, in c++11 it does), the file is not rewinded and the attributes are not read.

Solution: clear the eof bit.

Credits to Robert Schmidt for finding this and providing a solution.