Nope it was a new save. The crash happens when there're mons with the flag at false in party, regardless of the intended "starter" was distributed or not. I added an event in the player's room which gives a "special" ditto first activated and a non-special one every time after (actually copying...
Sloved! I commented out line 369 in the replacement Pscreen_Party.
# @startersprite.bitmap.dispose
Don't know why it works, but seems that the code tried to dispose something non-existent for a Pokémon without the "special" flag.
Strangely when I applied the script to a clean Essentials there's an error.
When I close the party screen and return to the pause menu, the following error message was displayed
I tried to look into the code but still no clues... Do you have any idea how it could be?
Sorry for the inconvenience!