Ok the error seems to be from this line
Error response received from backend. ServiceName=[OAuth], OperationName=[TokenGrantv2], Url=[<Redacted>], HttpStatus=[400], ErrorCode=[errors.com.epicgames.account.oauth.exchange_code_not_found], NumericErrorCode=[18057], ErrorMessage=[Sorry the exchange code you supplied was not found. It is possible that it was no longer valid], CorrId=[EOS-dcZHU7MtAUC0PThsUPEP5g-QMxWGWfQhEeHBMK8vBQqHg]
Try the following:
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Logging out of your epic games account in the Epic Games Launcher and then re-logging in
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Clear UEFN’s AppData, (Follow these Instructions https://www.epicgames.com/help/en-US/c-Category_Fortnite/c-Fortnite_TechnicalSupport/how-do-i-remove-the-fortnitegame-folder-from-localappdata-a000086962 ) Just instead of Deleting the FortniteGame folder Do it on the “UnrealEditorFortnite” Folder and just to be safe copy the folder in your desktop or something before deleting it, and then verify Both Fortnite AND UEFN from the Epic Games Launcher
Let me know if it helps