Helly was right, that bug could be ignored by just pressing ok on the error window and returning to the game, I assumed that was not the case since a lot of times I seem to be unable to get back into the game if I returned to Desktop (though there are times when this is possible).
Now I started the first chapter after ending the prologues and I really couldn't be happier about the game, so far it's everything I expected
As a curiosity, is there some Korean fansite with maps and other data about the game, like side quests? Is there some relevant missable content, like important story events you can only see if you go out of your way?
I'm quoting myself since I seem to have found an optional story event, with a rather lenghty scene of Sizz and Maria's life before leaving the island. It's apparently triggered by speaking with two NPC brothers near Dome's Inn when you go there at the beginning of Chapter 1, I'm not sure if it's really missable but I thought to point it out in case it is.
Another one triggered while speaking with a drunkard in Dome's bar, downstairs from Elluard's mansion. While the first dialogue could be mandatory since it concerns the Dallant, if you talk again with the same NPC (he's called Maverick, if I remember correctly) you will trigger yet another flashback of Sizz and Maria's life on the island. By the way, this event (the one when they try drinking wine for the first time) has an untranslated line, the only one I have seen so far.
I also noticed you can roam most of the Kingdom during Chapter 1's opening, including the capital city Baara and its cathedral, not to mention the port where Sizz landed during the prologue. Is there any point to it, aside from some interesting NPC dialogue? A thief was apparently plaguing Baara, but it didn't seem to be part of a subquest I could unlock at that time, so maybe it's actually foreshadowing for some story events further into the plot?
As a sidenote, the game seems to have a few lines breaking the fourth wall, like when an innkeeper says the party won't find another inn since RPG cities usually have just one, or when an hostile NPC asks Elluard why he dared to intrude his home and he says that he's only doing what his User wants.