Just for clarification, that error is pointing to the Custom Resolution script at this line:
return $game_map.priority_data[@x, @y] + z
In the demo, the error occurs after choosing to teleport from the crystal to the next map. I used a series of prints to see what values we're getting.
Quote
$game_map.priority_data.xsize = 50
$game_map.priority_data.ysize = 15
[@x, @y] (which are Game_Character variables) = [18, 26]
The 50 and 15 are indeed Map003's width and height. But the coordinates (18,26), which belong to the pink-haired girl on Map003, is after the maps have all been combined together.
So, from what I'm seeing, either the solution is to convert the Game_Character coordinates
temporarily to fit within the range of the priority_data table or combine all the maps' priority_data into one table. The latter sounds easier.