Reported on bug 796586, where a test file is attached: A QIF encoded in CP1252 can't be imported in Win10 because g_locale_to_utf8 uses the system character set and that's usually CP437 or CP850 for European languages, both of which use the codepoints > 0x80 for line-drawing symbols, not characters.
In version 4.4 of GNU Cash I could import QIF file exported from Quicken by converting the file to UTF-8 using iconv before import. I had similar issue using account and categories in ISO-8859-1 (latin1)