Double click the home button and "flick" the FM GO App up to make the App quit and close all files. Then re-open FM GO and your file and try again.
You might be interested in the script that I put in this file and set it to run "OnFirstWindowOpen". It detects this error state--which in my experience is only initiated while a file is opening, and tells you to restart the app in order to fix the problem:
I've recreated this many hundreds of times and I finally figured out what was happening... at least in my case. I went from a iPhone 5c to a iPhone 6. I kept getting that "being modified in another window" message and I was trying to figure out what was really different between the 5c and 6.
The iPhone 6 "Auto-Lock" feature defaults to 1 minute. Somehow the iPhone was telling FM Go that it was locked so you would get the error. My db would not let you use the camera, type any text into any field, select a date with a pop-up calendar or use a signature field. I changed the "Auto-Lock" (Located under Settings – General) to 5 minutes and the problem has gone away.
I need to do further testing, but at least I can use FM Go now and I have an area to keep my eye on. i hope this info helps.
Thank you for your post and information. I have passed your observations to Development and Testing hoping this will help with finding the cause.