This bug is because of incorrectly localized format string.
The original (English) format message is : Given %d experience to %s.
However it says "Format Error : %s에게 경험치 %d를 주었습니다" whose formats (%s, %d) are not in order so this format error occurs.

Others which reproduces that error :
English Korean
Given %d experience to %s %s에게 경험치 %d를 주었습니다
Given %d levels to %s %s에게 %d 레벨이 지급되었습니다
Taken %d levels from %s %s의 레벨을 %d 낮추었습니다