ECP on Exchange 2019 Server doesn't run without an Exchange 2016 Server

[GaryBurch]

I'm in the same situation as I'm sure many others are, migrating from on-prem Exchange 2016 to MS 365, then wanting to replace the on-prem Exchange 2016 Server with an Exchange 2019 Server just for running ECP, with no local mailboxes.

So far, I have:

• Migrated all the mailboxes in use to MS365
  • There are a fair few mailboxes that are no longer required, so haven't been migrated.  These are to be retired.
• Installed an Exchange 2019 Server on Windows Server 2022 (Server Core)
• Installed Certificates on the new Exchange 2019 server for webmail
• Changed DNS records for Exchange HTTPS services to point to just the new 2019 Server
• Run the HCW on the 2019 box to re-configure Hybrid Configuration to use the new server

Having done all of that, I've powered down the old Exchange 2016 server to test that everything works on just the new 2019 server.  It turns out that ECP is working when both servers are online, but as soon as the 2016 server is shut down, ECP (running from the 2019 server) stops working.  I can confirm that the 2019 server is serving the pages, as it's a new certificate than was used on the 2016 server.  The login page loads correctly, but after entering my credentials it returns HTTP 503 "<webserver> can't currently handle this request."  Once the 2016 server is brought back online it works again.

Is this expected behaviour because Exchange is unable to contact one of it's servers, that would disappear once it's cleanly decommissioned, or is there something wrong with the new server?  I've completely removed and re-installed Exchange on this box, but it doesn't seem to have made a difference.

[#2]

Hi @GaryBurch 

Did you move all the Arbitration/System and used user mailboxes form 2016 to 2019?
Also, you can check the backend certificate "Microsoft Exchange Server" on Exchange 2019 IIS on port 444. 

[#3]

I haven't moved any mailboxes to the 2019 server, I don't plan on even running a database on that server (or, do I have to keep arbitration mailboxes still).

I haven't done anything with the back end certificates on either server. What am I looking for on there, just that the cert is valid?

[#4]

@Thierry_Chapuisat The certificate on the Exchange Back End web site looks OK to me

Do I need to have a database and arbitration/system mailboxes on the on-prem server?  Or is it that because they exist somewhere they have to be reachable?

[#5]

@GaryBurch 

Mailboxes to access ECP must be moved in 2019.