Here is the error message I'm getting:

5x Prat-3.0-3.9.58/services/chatsections.lua:679: bad argument #2 to 'format' (string expected, got no value)
[string "=[C]"]: in function format' [string "@Prat-3.0-3.9.58/services/chatsections.lua"]:679: in function SplitChatMessage'
[string "@Prat-3.0-3.9.58/addon/addon.lua"]:642: in function <Prat-3.0/addon/addon.lua:616>
[string "=(tail call)"]: ?
[string "@Blizzard_ChatFrameBase/Mainline/ChatFrame.lua"]:3324: in function `ChatFrame_OnEvent'
[string "@Blizzard_ChatFrameBase/Mainline/FloatingChatFrame.lua"]:94: in function <...lizzard_ChatFrameBase/Mainline/FloatingChatFrame.lua:93>

Locals:
(*temporary) = "|cff40ff40|T2565243:16:16:0:0:64:64:4:60:4:60|t %s %s has earned the achievement |cffffff00|Hachievement:20118:Player-1426-0E62CA9A:1:8:22:24:4294967295:4294967295:4294967295:4294967295|h[The Isle of Dorn]|h|r!|r"
(*temporary) = ""
(*temporary) = "string expected, got no value"

I am also running XFaction which is the only other addon I use which affects the chat window. Not sure if that's related.

Give the newest version of XFaction a try and see if it's doing it