What reason could there be for a map scroll to not work?

I simply have ◆Scroll Map：Down, 100, 3 (Wait)
And nothing happens... some weeks ago it did, not sure what changed.

it would help if we could get the entire event, because it might be something else interfering.
the only ways to break that command would be if either the map is too small, or if a plugin broke the part of the engine containing that command.
so it is much more likely that it is something else that went wrong, especially since you never described what really happens with the rest of the event or how you try to trigger that command.

There's nothing else in the event...

wrong - an event is more than just its content.
we ask for screenshots of the events because that is the fastest way to get all the important event properties that might have been messed up

How big is your map, and what is the player's position prior to this event running?

It's big enough, in fact I calculated the exact number of tiles when it still worked, with the player at the right spot (at any rate from what I remember it doesn't not work if it exceeds, merely waits or goes to the next command)...

Do you know for sure the player is standing in the right spot? Add a sprite and keep the priority as Below Characters, just to make sure the event is still where you think it is. Or add a Show Text before the scroll command, just to make sure you're activating the event.

Yep, player could go over the sprite, and the text was displayed... just no scrolling.

If you change it to only scroll, like, 2 tiles, what happens?

Still nothing happens, no matter what number, speed too.

What if you try it in a brand new project, with a map the same size, and the player and event in the same places?

It isn't the event itself, so that points out to the possibility that the game engine is broken as the last of the options I mentioned above - something that could be tested like Shaz said with a new project.

If the same effect happens in a new project (unlikely in my opinion) then you should reinstall the editor because something damaged the files in newdata.

if the new project work, we need to see screenshots of your plugin manager to guess which plugin might be responsible for the problems.

It still doesn't work in a new project, and... there's no plugins too... this couldn't have anything to do with this, could it? As that's one thing that changed since it last functioned...

Ah, it was my forgetting to enlarge the new project why it didn't work... and I just found the conflicting plugin to be Galv's Cam Control MZ.

There's a plugin command in 'Galv's Cam Control MZ' to disable to the camera control, you need to use it if you want to use the normal map scroll to work. Check out the plugin documentation.

Try using Disable Focus plugin command I believe it is.