CAS not working with Outlook after OS upgrade

Copper Contributor



We have an on-prem Exchange 2010 SP3 environment. It consists of 2x Client Access Servers (CAS01+CAS02) and 2x Mailbox servers (MB01+MB02).

The CAS are running a Windows software NLB and the MB-servers are running a DAG, with CAS01 being the witness server.


3 of the servers are running Windows Server 2008 R2 while the last one (CAS01) earlier today was upgraded to Server 2012 R2 through an in-place upgrade due to the pending end-of-life of Server 2008(R2).


I know that both Exchange and OS versions are way behind, hence the upgrade :) a full migration to another Exchange environment is planned within the next 4-6 months, which is why we are only upgrading the server OS'es. Exchange 2010 goes EOL in October 2020, but the current setup should be decomissioned by then.



Now, the problem:

As stated, CAS01 was upgraded to Server 2012 R2 and the upgrade went as planned with no errors.
However, after the upgrade CAS01 no longer works with Outlook. OWA and ActiveSync works fine, but not Outlook.

All services are running, network connectivity is fine (obviously, since OWA and AS work) and everything else looks normal on the server.

When trying to start Outlook on a client machine I get the following error:
"Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opned. The attempt to log on to Microsoft Exchange has failed." (attached screenshot).
When getting this error, I have disabled the non-upgraded CAS02 in the NLB to ensure that CAS01 gets the requests.

When I disable CAS01 in the NLB and enable CAS02, Outlook works as normal.

The plan is to upgrade the remaining 3 servers to 2012 R2 as well, but this will of course wait until any and all problems are sorted.


I am by no means an Exchange guru, so I am running out of ideas on why this isnt working.

Any advice is greatly appreciated.


Best regards & thanks in advance

1 Reply

Update: the reason was very simple. OS upgrade is not supported on Exchange servers, end of story.

The solution would be to create new servers and migrate the environment.