Could be one of these global strings you're replacing, and even if it isn't I believe replacing the global is going to taint every blizzard function that uses it anywhere, which could cause other problems in the interface.
Specifically, if I had to guess, one of the quest strings you're replacing, and I strongly suspect this one..
Lua Code:
ERR_QUEST_REWARD_ITEM_MULT_IS = "|cffFFFF00+ %s |cffFFFF00(%d)";
Because the original value is this..
Lua Code:
ERR_QUEST_REWARD_ITEM_MULT_IS = "Received %d of item: %s.";
And you swapped the %d and %s, so it's trying to put a string where it's expecting a number.