Error occured in: Global
Count: 1
Message: bad argument #2 to '?' (Usage: self:SetParent([parent]))
Debug:
[string "=[C]"]: ?
[string "=[C]"]: SetParent()
[string "@Interface/AddOns/Skillet-Classic/UI/MainFrame.lua"]:459: ResetTradeSkillWindow()
[string "@Interface/AddOns/Skillet-Classic/Skillet.lua"]:1443: ShowTradeSkillWindow()
[string "@Interface/AddOns/Skillet-Classic/Skillet.lua"]:1255: SkilletShowWindow()
[string "@Interface/AddOns/Skillet-Classic/Skillet.lua"]:938: ?()
[string "@Interface/AddOns/AtlasLootClassic/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:109:
...sic/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua:109
[string "=[C]"]: ?
[string "@Interface/AddOns/AtlasLootClassic/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:19:
...sic/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua:15
[string "@Interface/AddOns/AtlasLootClassic/Libs/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:54: Fire()
[string "@Interface/AddOns/Attune/Libs/AceEvent-3.0/AceEvent-3.0.lua"]:120:
...ace/AddOns/Attune/Libs/AceEvent-3.0/AceEvent-3.0.lua:119

Not sure if a conflict with the other addons, but clicking on professions from skill book throws the above error.

@Dinoburt, I cannot reproduce this with the information you have given me. I believe this is either an addon conflict or a corrupted install. Please manually download from https://www.curseforge.com/wow/addons/skillet-classic/files and verify the MD5, delete your current ...\Addons\Skillet-Classic folder and reinstall. Uncheck "Load out of date AddOns" and let me know if you still have problems.

@Gunner-web, How do you open your professions? Do you use an action bar button? Do you use any modifier keys like Shift, Ctrl, or Alt when you click on the button?

Not sure if this is related but when I open a skill I only see the Blizzard "stock" screen instead of the skillet screens.

I've found from experimenting that following addons (every single one of them) are causing this reported behaviour:
Enchantrix (part of Auctioneer Suite)
EnchantrixBarker
MobInfo2
Missing TradeSkills List (TBC)

@Civic261, Thank you for that information. It will take a while for all addons to be updated and until they are, I can't resolve any conflicts with Skillet-Classic that might be caused by Skillet-Classic.

I only had to disable MTSL to make it work again, and I also have both Enchantrix and Mobinfo2 installed and enabled.
I have updated Enchantrix with the latest build from their website though.

Also, there is a fixed version of MTSL in the comments, it's only made to work with Wrath though, no new recipes added.

This should be fixed in Skillet-Classic-1.63.

Had encountered this in 1.61 also, and came to report it but seems to be working after updating to 1.63.