Somehow the font project is corrupt. Did you save and load it with the same FontCreator release? Do backups of your font project have the same problem?
yes, the same release.
The backups are o.k.
What is the exact version?
I understand. This looks like a rare issue, and it is most likely specific to FontCreator 15.
The best way forward is to upgrade to FontCreator 16, as it includes many improvements, new features, and several bug fixes. There is a good chance it will handle your font project correctly.
Please let us know if the problem still occurs in FontCreator 16.