The inability to create GedComs began at the same time as the sync-ing with my Ancestry Member Tree stopped, so I can only assume that there is some kind of connection between the two problems. (I had done both MANY times before this issue came up at the beginning of February of this year.)
I have just over 63K individuals in my file.
1) I don't have any problem creating a backup--I have no images in my file, so I can't say about those.
2) I just attempted an FTM export (to a different directory), but FTM closed immediately when I pressed the button to begin the export.
3) I just created a new file with one person, and attempted to create a GedCom. As soon as I pressed the button to create the GedCom, FTM closed immediately.
4) I saved the backup that I successfully created into a different folder, then opened it and attempted to create a GedCom--FTM immediately closed upon beginning that process.
Thanks for thinking about my problem!