Decomissioning and old Exchange 2013 server : OWA unavailable

Hello group, we are trying to decomission an old Exchange 2013 mailbox server.

Arbitration mailboxes are moved successfully.

However, when we offline the old Exchange Server prior to decommission in order to test for negative impact, we lose the access to ecp and owa, even with a working frontend CAS Exchange 2013 server and backend mailbox server remaining in production.

PS : We updated the virtual directories to Exchange 2013 version, if that information can be of any help.

[PS] C:\Windows\system32>Get-OwaVirtualDirectory -ShowBackEndVirtualDirectories

Name                                    Server                                  OwaVersion

----                                    ------                                  ----------

owa (Exchange Back End)                 NEWMAILBOXSRV                            Exchange2013

owa (Default Web Site)                  NEWCASSRV                                 Exchange2013

owa (Exchange Back End)                 OLDMAILBOXSRV                            Exchange2013

When OLDMAILMBOXSRV is offline, the requests fulfill authentication (auth.owa) page and stall here

We are going to check the IIS Logs for protocol status and win32 status and give you more information when we’ll try again to offline the server.

Any obvious thing we need to check in the meantime?