When a player has no more cards in his hand and is choosen as a target, is he than forced to discard 5 cards from his deck by the effect of Carcosa?

 Yes, you understand it correctly.  Carcosa says:

Action: Exhaust and pay 2 to choose a player. That player must either choose and discard 1 card from his hand or discard the top 5 cards of his deck.

In general, cards are taken literally.  If the player cannot choose 1 card from his hand, he can't do the first option.  Thus he would be forced to take the second option:  discard the top 5 cards of his deck.

 I think "choose" is important, you cant choose sth that has no legal target in this game. 
(In dominion you can, so its not about literal meaning but semantics of words in game rules).

AFAIK when card says "discard 2 cards" without choose; you have 1, you discard 1;
when it says "choose and discard 2" you cant choose 2 so you discard none or are forced to take other option)

 alr8 thank you very much

You Know Too Much works the same way. If a player has no characters in play, they must put 2 cards back from their hand on top of their deck. If they have no cards in hand they have to sacrifice a character. If they have 1 card in hand and no creatures in play, then you wasted a domain and should be ridiculed.