I also get an error message when I try to export my fonts.
This has only occurred since the update to 9.1, I have re-installed Font Creator, it makes no difference.
Kelvinch looks worse at small point sizes without hinting!
This issue is unrelated to the other thread. At least you have ttfautohint.exe installed.
I narrowed down the problem to one or more glyphs in the Mathematical Operators character set. If you delete the entire character set, the issue disappears.
If I delete these row of glyphs the problem also disappears.
The approxequal glyph is a composite of a composite. Making it simple solves the issue. So does making the “similar” glyph ($223C) simple while leaving the approxequal glyph as a composite.
There are other glyphs with a greater component depth so I don’t understand why that particular glyph should cause issues.
Thank you, I will look for composites of composites. There may be others, since discovering that overlapping composite contours are not a problem I have been using the characters as building blocks to build other characters, maybe this approach has some drawbacks. 
Font, Information, Max component Depth.
Just double-click that line, and it will be located in the font overview window.
OK, I admit it, the bug was in my font not in your software, it must have been around the time that the update came out that I “tidied up” the Mathematical Operators block. I only update the version of Kelvinch installed on my system at irregular intervals.
Erwin, it is a very nice touch to the interface, thank you, it saved me a lot of work, I have found all the composites of composites in Kelvinch Roman, the other fonts I will do tomorrow.
Thanks for the help.
There is no rule that says you cannot have a component depth greater than 1, and ttfautohint.exe can hint lots of other such glyphs without generating an error.
I have no idea why it fails on that particular glyph. It may be a bug in FontCreator or in the autohinter, but it’s not a bug in your font. As I said earlier:
That is correct.
The error message comes from the autohinter, so I suspect it is a bug in the autohinter.
No. I changed that glyph and it still failed, there were a couple of others. But at least it’s sorted out now and if it happens again I will understand the cause and how to solve it.
Thanks.