QuestieVersion: 6.5.1
...\Questie\Database\Corrections\QuestieCorrections.lua:161: attempt to index field 'itemData' (a nil value)

@TheJellly Try /questie -> Advanced -> Recompile database.

If that doesn't help remove the Questie folder and do a fresh install of v6.5.1

/questie won't open a window.

Try running this:
/run QuestieLoader:ImportModule("QuestieOptions").tabs.advanced:Initialize().args.questieReset.func()

Still won't open and following error message in chat Interface\AddOns\Questie\Modules\Libs\QuestieLib.lua:373: attempt to call global 'GetQuestLogTitle' (a nil value)!
Already fresh installed it 3 times

Fresh install doesn't help.
/questie won't open a window.

I also get a new error now.
Interface\AddOns\Questie\Modules\Libs\QuestieLib.lua:373: attempt to call global 'GetQuestLogTitle' (a nil value)

/questie won't open a window.

Try running this:
/run QuestieLoader:ImportModule("QuestieOptions").tabs.advanced:Initialize().args.questieReset.func()

@TheJellly Are playing TBC Classic or Classic Era? Note that we don't support private servers or any vanilla versions of WoW anymore.

Last resort is to remove the Questie.lua and Questie.lua.bak files you find in the WTF folder. This will reset all the Questie settings (won't touch completed quests).

Reopening ? Same error here.
CLassic Era

@nervusdm You have most likely TBC .zip from curseforge installed instead Era .zip
Could you show a screenshot of your Questie folder files? _classic_era_\Interface\AddOns\Questie\

After the screeshot, you could try to fix the thing by manually installing version GitHub has. It works on both TBC and Era+SoM unlike ones on CurseForge. https://github.com/Questie/Questie/releases/latest

You are right, I am sorry. I was using TBC...