Usually this error message pops up when trying to modify a calculation field.
So far I've checked all of the fields I'm trying to put data into and none of them are calculation fields. Is there anything I should be looking at in the file relationships section for either of the files?
You can also specify that a field is not modifiable in Field options so even a non-calculation field can trigger this error message.
Also, look at the match fields in your relationships for any that might be part of your layout. If you have "allow creation of records..." enabled for a relationship where the match field is a calculation field, you'll get this error if you enter data, say in the "add" row of a portal and FIleMaker tries to create a related record only to find that it can't enter data from the parent record into that calculation match field in the child record it is trying to create.
Finally, if all else is ruled out, run a recover on your converted file and see if any issues are reported. Even if none are reported, test the recovered copy to see if it still has this issue.