Open File uses an external data source reference to find the file to be opened.
Select the problem Open File script step in your script editor and then select "add filemaker datasource" from the Specify drop down in the lower right corner.
I found my problem.
One filename in each set had a "dot (.)" in it. The files have always worked OK in older versions of FMP. The file worked in FMP 12 except that the script didn't handle it.