Thank you for your post.
If you are getting that message when logging into the database file, then there is an issue either with the database file or FileMaker Go. Do you get the same message when trying to access as a guest via another desktop machine? If not, then delete FileMaker Go from your iOS device, and download a fresh copy of FileMaker Go from the App Store. Try again.
Keep me updated with any progress.
I deleted and reinstalled FM Go. Nothing else would work. Thanks for your advice.
This problem happens a lot on FM Go but never on FM Pro.