I came across this thread because my Mnemosyne was crashing (segfault) on editing cards. I am on macOS 10.13.3, using 2.6 it segfaults when I try to edit an entry. (Just using it to memorize and score works fine, funnily enough adding new cards works too.)
So I tested with 2.6-qt5.10 version, this one does not segfault but shows a way too big editing popup window with garbage white/grey stripes = editing does not work but app stays alive.
This is happening with a selfmade DB french/german vocabulary that grew to 1300 entries over the last 6month. After export in v2.x cards format and re-import (losing my scoring though) everything works fine again. I can send my segfault-causing ~/Libary/Mnemosyne folder if that helps.
br
mike