Thank you for your post.
The Import Records script step only supports FileMaker Pro files. Therefore, make sure you didn't specify another file type in the Import Records script step.
Lovely--problem solved in about 11 seconds!
I had left the Import Records step's File Type specification in its default setting of All Available, assuming (if I thought about it) that the file name's extension (".fmp12") would satisfy the FileMaker-only requirement.
Changing the setting to FileMaker Pro files solved the problem.