I've found these entries from an old FAQ (or Jedi Consular, I don't know):
"Q: How about this: Let's say you have a Rancor fighting two enemies. One enemy is 1 square away, and the other is 2 squares away, directly behind the first enemy. Can the Rancor attack the enemy in back?
A: No. The more distant enemy would be considered to have cover from the closer enemy, and (obviously) the more distant enemy is not the closest. Therefore, it would not be a legal target according to the targeting rules.
Q: Another cover question: What if a Rancor is fighting an enemy 2 squares away on a low objects square. Does that enemy get cover?
A: Yes. The Rancor is not literally adjacent to the low objects, so he cannot ignore them."
These rules are up to date?
Or there is no cover when I attack an enemy character with Melee Reach?