Yes, that's what I wanted to see. Looking at that, I can't see how case "Party_begins" is triggered but I'm guessing it's by dialog with Toni Haraldo. (It can be helpful to put comments on such lines, e.g. case "Party_begins": // Triggered by dialog with Toni Haraldo so that it's clear to anyone reading the code, including yourself in the future, how the quest progresses.)
So, "Party_begins" teleports you to "Wedding_Ballroom", locator "reload1", places a lot of characters in the room, and triggers "Party_begins2". You talk to yourself, presumably triggering "Party_begins3", which sets a trigger for "Dont_leave" by you being on "reload1", which probably activates at once because you've just teleported onto it. I'd be inclined to have "Party_begins3" instead set a trigger on a locator somewhere else in the room for, say, "Party_begins4", and have the trigger for "Dont_leave" there. That means you have to move away from the door to trigger "Party_begins4", and then move back to the door to trigger "Dont_leave".
"Dont_leave" is Puerta Velador coming to talk to you, presumably triggering "Puerta_returns", and that's where the trigger on locator "box1" is set.
Does Puerta Velador in fact talk to you when you play this? If so, what happens if you move to the box after the dialog finishes?
Try editing "PROGRAM\InternalSettings.h". Go to the bottom and set VISIBLE_LOCATORS to 1. Then play that scene and check that locator "box1" is where it's supposed to be.
And then, after you have either talked to Puerta Velador or given up on the dialog starting, have a look at "compile.log" - or upload it here. You should be able to find lines in there showing which quest cases have activated.