I was using FontCreator in version 15(.0.0.2951) and opened a font file of WOFF2 file format. When I was ready to close it, the file cannot be closed and it reported an error like this pitcure below:
The font file was not edited at that time.
https://www.high-logic.com/tmp/fontcreator/FontCreatorSetup15.0.0.2954-x64.exe
Can you try this release and let me know your results?
Problem solved, thanks for your great work!