I have this failure:
WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request)
at System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request)
at Microsoft.UpdateServices.ServerSync.ServerSyncCompressionProxy.GetWebResponse(WebRequest webRequest)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
at Microsoft.UpdateServices.ServerSyncWebServices.ServerSync.ServerSyncProxy.GetAuthConfig()
at Microsoft.UpdateServices.ServerSync.ServerSyncLib.InternetGetServerAuthConfig(ServerSyncProxy proxy, WebServiceCommunicationHelper webServiceHelper)
at Microsoft.UpdateServices.ServerSync.ServerSyncLib.Authenticate(AuthorizationManager authorizationManager, Boolean checkExpiration, ServerSyncProxy proxy, Cookie cookie, WebServiceCommunicationHelper webServiceHelper)
at Microsoft.UpdateServices.ServerSync.CatalogSyncAgentCore.SyncConfigUpdatesFromUSS()
at Microsoft.UpdateServices.Serve
Which looks similar-enough to post on this old thread.
The last successful sync I saw was 4.56pm 20210921 AEST (UTC+10) first unsuccessful was 4.56pm 20210922 AEST
In UTC, that is the breakage happened sometime between 20210921 10:56 and 20210922 10:56.
Nothing but failure since, and I have just now (20210922 23:04 UTC)tried a manual sync, which has failed