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File maker does not reference fields by their name except in very specific cases. Each time that you create a new field, table or table occurrence, that newly created object is assigned a unique ID. FileMaker uses those IDs to reference the data. Most such errors occur when the front end is set to reference a field using an ID that does not exist in the back end file.
Note that if you add 5 fields to a back end file's table, then open a different copy of the same back end and make the same changes, it's very, very easy to get some fields that don't have exactly the same internal ID's. Then a "Front end" file that was correctly referencing data with one of the two back end files can get these errors with the other back end file if it is used in place of the first.
What would be your advice for the future when needing to adding another 'module' to an existing solution?
For example, one that has an 'Orders' module that has tables for parts, customers, addresses orders and order line items. An additional iPad based module is required to record jobs and services that would share parts customers and addresses, but would also have tables for jobs and work history.
Keep just one version of the back end in place. That back end might consist of several files, but manage them to be one version.
Then use that one one version as the context for any updates to the design of your front end files.