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One user reported that turning off his VPN then installing the extension from the Epic Webstore worked. So please make sure Epic's encrypted proxy and any VPN on your system are all turned off before installing an extension.
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Actually I think this is the issue. I tested it and it both created the issue then solved the issue for me. If you have Downloads set to "Ask me where to save" in Epic's Preferences/Settings, for some reason extensions won't install. Please uncheck that, then install the extensions, then you can re-check that setting.
I'm not sure why the extensions don't install when "Ask me where to download files to" is checked, but that seems to be interfering with extension installation.
You may also need to disable a VPN or Proxy -- but so far, the encrypted proxy did not interfere with installing extensions in our testing, only that download location setting did.
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