I had this same issue.. Make sure the FM DB is the exact case as the config file. a simple lack of a capital letter in the database name was my issue..
I have done a number of FIAS Apps and never seen this issue. It must be a setting or configuration as was recommended.
What I found out some time ago was that the FISA build got corrupted. If you exit Xcode, and then later reenter you probably will not see this kind of error. But, once in awhile you'll note that the name for your FileMaker file is replaced with an unassigned name. If you don't immediately exit Xcode, and if you continue working it will corrupt the build. There is no undue, you're basically screwed and have to start all over again and create a new app ID and the developer account.
I just changed the name slightly and rebuilt the app, then built the FISA file and continued.
Important: to keep this from happening again, and you see an unassigned name and Xcode for your project stop immediately. Close Xcode, then reopen it from the project in your FISA built file. It will replace the unassigned name and replace it with the correct name. At this point you can continue your development and should not see a fail to load when you run the simulator. I did create a bug report and sent to FileMaker. I don't know what action they took to notify the community about this issue?