Do you have FileMaker Advanced?
I'd generate a Database Design Report ( DDR ) and do a text based search for that script step. (Or use one of the available developer tools to search for it.
You can also take careful note of when the error message appears. This should enable you to make some decent guesses as to which script might contain the problem script step in order to check for them.
You can also insert some show custom dialog steps into suspect scripts to see which messages appear before you get this error. If you can identify the script and it doesn't have this step, you can try replacing the script with a new copy with the same steps (rewrite it from scratch) and see if that makes a difference.
It must be in there somewhere. Usually these are on a layout trigger. Check the DDR as well.