Recently I had to install an Exchange 2016 CU16 server as part of an Exchange 2010 migration project. When the server was running as I started moving mailboxes to it, I noticed an increasing number (linear to the number of users it seems) of event ID 1 MSExchange Autodiscover errors in the Application Eventlog of the server:

Unhandled Exception “Object reference not set to an instance of an object.”
Stack trace: at Microsoft.Exchange.AutoDiscoverV2.FlightSettingRepository.
GetHostNameFromVdir(ADObjectId serverSiteId, String protocol) at Microsoft.Exchange.AutoDiscoverV2. AutoDiscoverV2.ExecuteOnPremEndFlow(AutoDiscoverV2Request request)
at Microsoft.Exchange.AutoDiscoverV2.AutoDiscoverV2.Execute (AutoDiscoverV2Request request, ITenantRepository tenantRepository) at Microsoft.Exchange.AutoDiscoverV2.AutoDiscoverV2HandlerBase.<>c__DisplayClass11_0.b__0() at Microsoft.Exchange.Common.IL.ILUtil.DoTryFilterCatch(Action tryDelegate, Func`2 filterDelegate, Action`1 catchDelegate)

This issue was resolved in Exchange 2016 CU15 en Exchange 2019 CU4, but occasionally pops up in newer versions, and unfortunately on my Exchange 2016 CU16 server.

Although I don’t know the cause of this the workaround is simple. Set the ExternalURL property of the Autodiscover virtual directory, like this:

Get-AutodiscoverVirtualDirectory -Server | Set-AutodiscoverVirtualDirectory -ExternalUrl https://autodiscover.contoso.com/Autodiscover/Autodiscover.xml

This was (is) a known issue at Microsoft and I brought this again to their attention. When something develops, I’ll update this blogpost.