
Hi @AlexRobert ,
You have to allow WNS Traffic. For the best user experience, SharePoint Server 2019 will send change notifications to sync apps via WNS web push notifications. This feature ensures sync users quickly have the latest copies of any SharePoint Server file updates. You may need to take steps to ensure outbound requests from your SharePoint Server and your users' computers can reach and properly interact with the WNS service.
For Reference:
Configure syncing with the new OneDrive sync app
Set up Microsoft OneDrive in a SharePoint Server on-premises environment
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