In SpecialWatchlist::cutoffselector(), values like 1/24 or 6/24 are cast to string via strval(). However, in some locales (e.g. $wgShellLocale = 'ru_RU.utf8';) strval will return a localized form of the number, e.g. "0,04" instead of "0.04".
Later, when this "0,04" is used in arithmetic operations, it's treated as "0". (and PHP warning "A non well formed numeric value encountered" is shown).