Why did Morden ask the Shadow's question to Kosh? They knew perfectly each other. Was it an error from Morden or was it a way to say 'hi. We are going to try it again after the last war'?
He did not. If you are thinking that was what happened when Kosh and Morden met in Signs and Portents, you misremember/misinterpreted the scene. Morden was trying to avoid Kosh, and successfully did so earlier in the episode. Kosh was confronting Morden, trying to get him off the station and preventing him from meeting the command staff (notice he never asked Sinclair), the implication is there was even a fight after the scene cut off, with dialogue later in the episode mentioning damage to Kosh's encouter suit.