xchange 2013 Activesync not working receiving 401.1 in IIS logs ExRCA site test results are succesfull

Hi,

We currently have 2 servers on with Exchange 2010 and one with 2013 in a Coexistence config, so far most things seem to work except Activesync.

The problem arises when I try to connect a new device to an account with a mailbox on the new server, I can see that the device gets neglected with the following error in IIS logs SAMSUNG-GT-I9300/101.403 - 401 1 1326 265 

My current settings on the respective server can be summarized to the following:

2010 CAS:

InternalUrl                                : https://webmail.mydomainname.com/Microsoft-Server-ActiveSync

ExternalUrl    

BasicAuthEnabled                           : True
WindowsAuthEnabled                         : False

2013 CAS

InternalUrl                                : https://webmail.mydomainname.com/Microsoft-Server-ActiveSync

ExternalUrl     : https://webmail.mydomainname.com/Microsoft-Server-ActiveSync

BasicAuthEnabled                           : True
WindowsAuthEnabled                         : False

Owa is working properly for all mailboxes and outlook is working almost perfect, except that the users need to fill in credentials once after mailboxes have been migrated to the 2013 server (NTLM is configured for both internal and external clients) and CAS 2010 have basic and NTLM under IISAuthenticationMethods) 

I have run both the Exchange Activesync and Exchange activeync autodiscover tests witht the ExRCA and both test succeds and I can see succesfull connection attempts in the IIS log.

I would really appreciate some troubleshooting tips and tricks,

BRgds 

Cris