We give some new regularity conditions for Fenchel
duality in separated locally convex vector spaces, written in
terms of the notion of quasi interior and quasi-relative interior,
respectively. We provide also an example of a convex optimization
problem for which the classical generalized interior-point
conditions given so far in the literature cannot be applied, while
the one given by us is applicable. Using a technique developed by
Magnanti, we derive some duality results for the optimization
problem with cone inequality constraints and its Lagrange dual
problem and we show that a duality result recently given in the
literature for this pair of problems is incorrect.