The fix for build_constraint_matrix() works... I need no more to enforce nodal mismatch and the solution doesn't blow up (I use max_h_level = 2).
The function limit_level_mismatch_at_edge(...) included  in the CVS doesn't work... I'm trying something else but I don't find a solution that works for all the type of elements (P1, P2, ecc...). I don't know if  it is still useful, but probably it could be used with max_mismatch = 2.
Thanks all for the help!

Lorenzo