If you’re seeing the 0xE0000784 error code, OneNote can’t use the WebClient service on your computer to access your notebook.
Do the following:
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On your keyboard, press WINDOWS+R.
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In the Run dialog box, type services.msc, and then press Enter.
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In the Services window, scroll down to the WebClient service in the Name column and look at the current setting in the Startup Type column. If it isn't set to Automatic or Manual, do the following:
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Right-click WebClient, and then click Properties.
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In the WebClient Properties window, change the Startup Type to Automatic, and then click OK.
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Restart your computer after changing this setting.
More help
If you’re still seeing this error message after troubleshooting it, please report this issue in the OneNote forums on Microsoft Answers so we can investigate the cause and find a solution.