if it's really caused by replacing that global string, you could also try something that is iirc not part of standard Lua but included in WoW: changing the arguments' order
since the original string has the amount before the item's name, that's the order the arguments are passed to format. So try:
Lua Code:
ERR_QUEST_REWARD_ITEM_MULT_IS = "|cffFFFF00+ %2$s |cffFFFF00(%1$d)";
That should change the order in which the arguments are inserted.