Sorry for the doublepost but i found a solution for my problem. There is a certain part in the script which caused the error:
# true: Use actor's walking graphic.
# false: Don't use actor's walking graphic.
# If false, battler file with "_1" is required since walking file is not used.
# "_1" and subsequent files ("_2", "_3", etc.) should be uniform in size.
WALK_ANIME = true
This was on false for some reason, and that is why the maker wanted another walking graphic with a _1 at the end. Really stupid -.-. But oh well, now it works fine like before ^^.