Outlook constantly prompting for password

Hi All,

We are currently in co-existence between exchange 2010 and 2013.  Yesterday I did an iisreset on our exchange 2010 and 2013 servers and after the restart all users with mailboxes on the 2013 server are now being prompted for credentials when opening outlook.

I spent some time troubleshooting the issue, i checked the autodiscover authentication and both the 2010 and 2013 servers are set to NTLM. I modified the authentication priority within iis by changing NTLM to be in first position within Providers for Windows Authentication, unfortunately this did not help.

Within powershell I ran a:

Get-OutlookProvider

and got the below output:

Name                                  Server                                CertPrincipalName                    TTL
----                                  ------                                -----------------                    ---
EXCH                                               1
EXPR                                              1
WEB                                                                                                             1

I then ran a: 

Set-OutlookProvider -Server SERVER EXCH -CertPrincipalName msstd:SERVER.DOAMIN.com.au -OutlookProviderFlags:none

Name                                  Server                                CertPrincipalName                    TTL
----                                  ------                                -----------------                    ---
EXCH                                  SERVER                                 msstd:server.domain.com.au             1
EXPR                                  SERVER                                 msstd:server.domain.com.au            1
WEB                                                                                                             1

Unfortunately this did not fix the issue either.

I inspected the 'Microsoft Exchange Proxy Settings' within outlook and unticked the 'On fast networks, connect using HTTP first, then connect using TCP/IP'. Once i made this change Outlook stopped prompting for credentials. See below screenshot:

NOTE: I have remove the server and domain information, sections are greyed out as the screenshot was taken after putting the GPO in place.

I have now put a band aid solution in place via group policy to untick this setting, everything is now working as to be expected. I suspect there is a misconfiguration with the Autodiscover but i'm not 100% sure.

Does anyone have any ideas?