Serafettin had a bunch of glyphs with self-intersection problems, and these were causing FontForge to crash on the Expand Stroke operation. Because Serafettin uses scripts to build the different weights, it was hard to see where the problem was until I made FontForge a lot more verbose about which glyphs it was processing.
Even so, if a glyph self-intersects, it might be impossible to spot how the intersection is happening until you zoom right in and see that what you thought was a sharp corner turns out to be a little twisted triangle of points. If the points are right on top of each other, you won’t see the problem at any zoom level, so you’ll need to trust the Simplify operation.
After all that work, I now have another FontForge bug to report. Try typing a some text ending in ‘+’, like ‘GPLv2+’ into the the TTF name fields. Save your work and observe that the UTF-7 (yes -7, not -8) encoding of the SFD turns this into ‘GPLv2+-‘. Now read in the SFD and see that your license field says ‘GPLv2+-‘, and subsequent saves will add another ‘-‘ every time. Boo, hiss.