After replacing certificate we saw this on EX2019 DB box:

Log Name: Application
Source: MSExchange Front End HTTP Proxy
Date: 22/12/2021 13:08:39
Event ID: 2001
Task Category: Core
Level: Error
Keywords: Classic
User: N/A
Computer: exserver.domain.local
Description:
[Owa] Failed to load SSL certificate.
Event Xml:



2001
2
1
0x80000000000000

3495586
Application
exserver.domain.local



Owa



Log Name: Application
Source: MSExchange Front End HTTP Proxy
Date: 22/12/2021 13:08:59
Event ID: 2001
Task Category: Core
Level: Error
Keywords: Classic
User: N/A
Computer: exserver.domain.local
Description:
[Ecp] Failed to load SSL certificate.
Event Xml:



2001
2
1
0x80000000000000

3495590
Application
exserver.domain.local



Ecp

OK so there were a few issues here, firstly the Exchange Back End website had lost it's certificate association which stopped all comms with the Exchange server including powershell:



And so we could not change anything.

We had to repair the association with the site in the IIS manager bindings:







Once the certificate was reinstated, we ran IISReset on the server and then we regained access to Exchange PowerShell

We could then re-associate all of the services with the correct certificate using:

Enable-ExchangeCertificate -Thumbprint [Thumbprint] -Services POP,IMAP,IIS,SMTP

All we had to do after that was to re-associate the relationship with our edge-transport server as we use one, but otherwise you should be done. 🖐