Currently playing Iorveth's path in Chapter 2. To complete the dice poker quest for this chapter, you need to beat five people. Problem is, whenever I try to challenge Hagard (the fifth person) the screen flashes black for a moment, and instead of going to the dice poker minigame, Hagard acts like I just played him and lost. This happens no matter how many times I talk to him, and the same thing happens even if I try to challenge him from an earlier save.

Is this a known issue by any chance? My version is updated to the latest patch (EE + 3.1), so I'm wondering if the bug is related to that somehow.

Yep, corrupted save. Had to reload a lot earlier than I really wanted to, but the game is winnable now.

Ehm..is there any other way? I cannot go back to a earlier save. Is there a file I can edit, or a console command to trigger the right behaviour?

I found another solution worth trying perhaps.

It seems that it is repro only when Hagart is in his house at night, sleeping. I meditated till the morning and spoke to him outside.

It worked :)

Glad you got it worked out, but seriously—keep old saves. Save corruption is rare, but often the only way to get around certain problems is to go back to an earlier save. Best not to tempt fate.

I usually keep only the last 10 or so saves. The thing is that I usually hate replaying parts of the games in order to pass "bug" points ;)

Had the same issue. I doubt it's savegame corruption.

Indeed, this worked for me also... You have to ask him for a game when he's outside during the day. When he was inside I had the same issue as the OP. Nothing to do with corrupt save.