The 0xE000078B error code means OneNote is having trouble communicating with the web server where your notebook is stored. Most likely, there's been a server timeout, which occurs when the server is busy and can't respond to your requests.
OneDrive
If you’re syncing notes to OneDrive, try syncing your notes again in a few minutes. If you suspect a service outage, you can check OneDrive status at https://portal.office.com/servicestatus.
SharePoint
If you’re syncing notes to SharePoint, try syncing your notes again in a few minutes or talk to your server administrator for help resolving this issue.
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.
