Bogran´s special ability says that he gets +2 damage against every target he did not have LoS at the beginning of his turn.

Is the LoS here actually meant to be calculated like for an attack or alternatively like for purposes of spawning, ignoring other figures?

Probably someone remembers an older ruling form the ancient forums?

"When Bogran the Shadow attacks an enemy that did not have line of sight to him at the start of the current turn, he gains +2 damage."

I don't see any indication that line of sight should be calculated differently than for a standard attack.

I agree. It has nothing to do with the special rules when the overlord is spawning new monsters and line of sight of the heroes goes "through" enemy figures.

+1 to each. The los rules for spawning are specifically noted as an exception. Unless something states explicitly that it would also follow that exception, I would treat as LOS is treated in all other circumstances. If "the Shadow" is behind other hero figures or even other enemy figures that block LOS to his target before his turn, he would get that bonus. Additionally, this applies for each attack made in the cases of battle w/fatigue, rapid fire, etc.

In fact, on further reflection I think the entire question is moot. LOS is usally thought of as reciprocal (if figure A can see figure B, figure B must be able to see figure A), but in the case of spawning monsters, Hero LOS passes through monsters. However, by the wording on Bogran's card it is monster LOS that matters for the attack bonus, and even during spawning that remains unchanged. Either way you rule it, the effect is the same.

Yep, it's basically a "thief's backstab".

Bogran starts his turn in front of a closed door, opens it, surprises the beastman in the new revealed area and move for the kill. Or Bogran turns around the corner, move around the rubbles and so on.

I was never tempted to use this Hero anyway.