I did write another program to check the file, and in the whole file there was only a single UTF character (code point U+00E8) that was not classic ASCII (code points <= 127), but removing the character still resulted in a crash.
It might have been the mode line comment itself was malformed according to MIT Scheme's rules, which might have subtle differences from Emacs. I asked my question on the MIT Scheme users mailing list.
