The latest World of Warcraft patch has made patching unreliable at best.
Currently, we have two workarounds that should make it possible to get back to gaming while we address the issue.
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Go to Options->Downloader preferences and disable "Enable Peer-to-Peer Transfer". You may have to do this quickly before the launcher crashes, although the UI remains useful after the crash in most cases. If it still crashes try to restart the launcher and see if it works.
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Quit the process completely (ensure that "launcher.exe" is not running in the background. Then, restart. Over and over again.
Our sincerest apologies for the inconvenience and many thanks in advance for your patience while we triage this issue!