Azure AD Connect V1.X versions no longer support the V2 endpoint

Microsoft introduced the Azure AD Connect sync V2 endpoint with version 1.6.4.0 in March 2021. Among the improvements, the V2 endpoint includes performance improvements and allows for synchronization of groups with up to 250K members. Enterprise customers with groups of 50K or more were encouraged to move to the new V2 endpoint.

AAD Connect version 2.0.3.0 was released in July 2021 and was a major upgrade. It supports the V2 endpoint by default, but requires Windows Server 2016 or 2019 due to it's dependency on SQL Server Express 2019 for localDB. There are still many customers running AADC V1.x for this reason.

Today, Microsoft updated the AADC version history to say that the V2 endpoint is no longer available for V1.x versions. 

UPDATE - 11/10/2021: Microsoft just added the following information to the AAD Connect version history:

Known Issues

There is an issue where customers who have the V2 endpoint running with an older version and try to upgrade to a newer V1.6 release will see that the 50K limitation on group membership is reinstated. We will not fix this issue in V1.6 and require customers to upgrade to AADConnect V2.0 if this is an issue for them.

Azure AD Connect V1.x customers are strongly encouraged to update to V2.x, keeping in mind that this may require installing AADC V2.x on a new Windows 2016 or Windows 2019 server. I wrote a step-by-step article on upgrading here.

In the meantime, if you are still using Azure AD Connect 1.x you should make sure you're using the V1 endpoint using the following steps.

First, check to see which sync endpoint you're using with these cmdlets, run from the server where Azure AD Connect 1.x is running:

• Import-Module 'C:\Program Files\Microsoft Azure AD Sync\Extensions\AADConnector.psm1'
• Get-ADSyncAADConnectorExportApiVersion
• Get-ADSyncAADConnectorImportApiVersion

If both Get-* cmdlets return the value "1", you're using the V1 endpoint. Nothing more to do here, except plan to upgrade to AAD Connect V2.x as soon as reasonable.

If the values returned are "2", you're using the V2 endpoint and need to change it back to V1.

• Import-Module 'C:\Program Files\Microsoft Azure AD Sync\Extensions\AADConnector.psm1'
• Set-ADSyncScheduler -SyncCycleEnabled $false
• Set-ADSyncAADConnectorExportApiVersion 1
• Set-ADSyncAADConnectorImportApiVersion 1
• Set-ADSyncScheduler -SyncCycleEnabled $true

Be aware that the V1 endpoint cannot sync groups with 50K+ members. You should plan to upgrade to AAD Connect V2.x as soon as possible.