Most likely reasons for this are:
1. The script was imported from somewhere and referred to field(s) that don't exist in the current file
2. Field(s) have been deleted or renamed after the script was created
Check both these possibilities before doing any further worrying.
This is intended behaviour/feature of FileMaker. FileMaker is very good at tracking the fields. When you rename a field, FileMaker updates it in scripts, relationships and calculations. What you see is the field which has been deleted, so can not be referenced anymore.
You will need to replace it with the a new field. If you can, go back to the backups to find out what was the field which was deleted.
On the other hand, since you do not report any problem with the script, these script steps could be not relevant anymore. You will need to check the script logic and probably remove the script steps referencing the missing fields.
Yes, quite so nicolai. My mistake on renaming of fields. A further thought is that the script steps leading up to the missing field step(s) may give you a clue as to which field was there originally. This doesn't always work, mind you, with an unfamiliar script—even one you wrote yourself and haven't reviewed in ages, in my experience!
What's confusing is that the script references 4 fields according to the development report, but only 3 are present in the script. The script works fine. I don't understand why the DDR report is off, but it's off. Anyone else experienced this?