I get the following error whenever I log into a toon on TBCC

3x AddOnSkins\Core\API.lua:900: attempt to index local 'frame' (a nil value)
[string "@AddOnSkins\Core\API.lua"]:900: in function SkinFrame' [string "@AddOnSkins\Skins\TBC\Blizzard\Mail.lua"]:42: in function <AddOnSkins\Skins\TBC\Blizzard\Mail.lua:3> [string "@AddOnSkins\Core\Core.lua"]:215: in function <AddOnSkins\Core\Core.lua:215> [string "=[C]"]: in function xpcall'
[string "@AddOnSkins\Core\Core.lua"]:215: in function CallSkin' [string "@AddOnSkins\Core\Core.lua"]:305: in function ?'
[string "@AddOnSkins\Libs\Ace3\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:119: in function <...ibs\Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:119>
[string "=[C]"]: ?
[string "@AddOnSkins\Libs\Ace3\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:29: in function <...ibs\Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:25>
[string "@AddOnSkins\Libs\Ace3\CallbackHandler-1.0\CallbackHandler-1.0-7.lua"]:64: in function `Fire'
[string "@AddOnSkins\Libs\Ace3\AceEvent-3.0\AceEvent-3.0-4.lua"]:120: in function <...s\AddOnSkins\Libs\Ace3\AceEvent-3.0\AceEvent-3.0.lua:119>

Mail.lua.gz
this is my mail.lua, still not finished but I dont get any errors... maybe its related to an addon which modifies your mailbox?

just copy it to _classic_\Interface\AddOns\AddOnSkins\Skins\TBC\Blizzard

That seems to work fine. I use TSM btw which does modify mailbox, but Ive logged a few toons checked mailboxes and used the AH and no errors at all.

if you don't use the blizzard mailbox at all, you can disable it in the options:

but yeah, the blizzard skins really need some love. especially on TBC.