@Birdsteeth sorry for the late reply, that error indicates an incomplete download/extraction. Things you can try would be
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Redownload new core from mirror or torrent linked in this post (assuming you’re on windows since this thread is about launcher)
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Make sure the extraction program you’re using is up-to-date.
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Using a different extraction program, (WinZip or 7-Zip and if neither works, then try WinRar or others).
If you’re on windows, the save file is located in a different directory so you don’t need to transfer it manually.
@RandomAI Try the steps above