How to move Azure AD Connect to a new server, when the old server is unavailable?

Question

Moving Azure (Entra) Sync 2.3.20 to a new PC after old machine died. No backups available on old machine. New machine is Windows server 2019, connect is latest version. Old machine ran older sync tool.

Software installed, no problem. Gave it tenant admin credentials, it connected. Gave it local AD admin credentials, it connected.

It fails during the AAD Sync. Here's the error (i changed the domain to contoso):

[09:09:58.628] [ 40] [ERROR] Creation of connector contoso.onmicrosoft.com - AAD failed. This may be due to replication delay. Retrying after 5 seconds ...

Exception Data (Raw): System.Management.Automation.CmdletInvocationException: An error occurred while sending the request. ---> Microsoft.IdentityManagement.PowerShell.ObjectModel.SynchronizationConfigurationValidationException: An error occurred while sending the request.

at Microsoft.DirectoryServices.MetadirectoryServices.UI.WebServices.MMSWebService.ValidateConfigurationParameters(Connector connector)

at Microsoft.DirectoryServices.MetadirectoryServices.UI.WebServices.MMSWebService.CreateConnector(Connector connector, Boolean validate)

at Microsoft.IdentityManagement.PowerShell.Cmdlet.AddADSyncConnectorCmdlet.ProcessRecord()

--- End of inner exception stack trace ---

at System.Management.Automation.Runspaces.PipelineBase.Invoke(IEnumerable input)

at System.Management.Automation.PowerShell.Worker.ConstructPipelineAndDoWork(Runspace rs, Boolean performSyncInvoke)

at System.Management.Automation.PowerShell.Worker.CreateRunspaceIfNeededAndDoWork(Runspace rsToUse, Boolean isSync)

at System.Management.Automation.PowerShell.CoreInvokeHelperTInput,TOutput

at System.Management.Automation.PowerShell.CoreInvokeTInput,TOutput

at System.Management.Automation.PowerShell.Invoke(IEnumerable input, PSInvocationSettings settings)

at Microsoft.Online.Deployment.PowerShell.LocalPowerShell.Invoke()

at Microsoft.Online.Deployment.PowerShell.PowerShellAdapter.TypeDependencies.InvokePowerShell(IPowerShell powerShell)

at Microsoft.Online.Deployment.PowerShell.PowerShellAdapter.InvokePowerShellCommand(String commandName, InitialSessionState initialSessionState, IDictionary`2 commandParameters, Boolean isScript)

at Microsoft.Azure.ActiveDirectory.Synchronization.PowerShellConfigAdapter.ConnectorConfigAdapter.AddConnector(Connector connector)

at Microsoft.Azure.ActiveDirectory.Synchronization.Config.ConnectorAdapterBase.CreateOrUpdateConnectorCore()

at Microsoft.Azure.ActiveDirectory.Synchronization.Framework.ActionExecutor.Execute(Action action, String description)

at Microsoft.Azure.ActiveDirectory.Synchronization.Config.ConnectorAdapterBase.CreateOrUpdateConnector(IEnumerable1 objectClassInclusions, IEnumerable1 attributeNameInclusions, ParameterKeyedCollection connectorGlobalParameters, Boolean createRunProfile)

at Microsoft.Online.Deployment.Types.Providers.SyncDataProvider.CreateConnectorWithRetry(ConnectorAdapterBase connectorAdapter, IEnumerable1 objectClassInclusions, IEnumerable1 attributeNameInclusions, ParameterKeyedCollection connectorGlobalParameters, Boolean createRunProfile)