Exchange 2010 Mailbox Users Cannot Access 2013 Shared Mailboxes

Hi Guys & Girls,

I've a painful issue with my environment which is made up of 1 x 2010 CAS server with 1x 2010 Mailbox server and 1x 2013 CAS server with 1 x 2013 mailbox server.

The users who've yet to be migrated to 2013 and are still on the 2010 server are unable to open shared mailboxes which have already been migrated to 2013.

When trying to access shared mailboxes on the 2013 servers they received the following error:

The configuration of OutlookAnywhere on my two CAS servers is as follows:

RunspaceId                         : aed28f13-cdd2-4dcf-a0a8-96b8e2518171
ServerName                         : THCAS1
SSLOffloading                      : True
ExternalHostname                   : mail.resourcegroup.co.uk
InternalHostname                   : mail.resourcegroup.co.uk
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Ntlm}
XropUrl                            :
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : False
MetabasePath                       : IIS://THCAS1.ResourceGroup.co.uk/W3SVC/1/ROOT/Rpc
Path                               : C:\Windows\System32\RpcProxy
ExtendedProtectionTokenChecking    : None
ExtendedProtectionFlags            : {}
ExtendedProtectionSPNList          : {}
AdminDisplayVersion                : Version 14.3 (Build 123.4)
Server                             : THCAS1
AdminDisplayName                   :
ExchangeVersion                    : 0.10 (14.0.100.0)
Name                               : Rpc (Default Web Site)
DistinguishedName                  : CN=Rpc (Default Web Site),CN=HTTP,CN=Protocols,CN=THCAS1,CN=Servers,CN=Exchange
                                     Administrative Group (FYDIBOHF23SPDLT),CN=Administrative
                                     Groups,CN=ResourceGroup,CN=Microsoft
                                     Exchange,CN=Services,CN=Configuration,DC=ResourceGroup,DC=co,DC=uk
Identity                           : THCAS1\Rpc (Default Web Site)
Guid                               : 71f76f2a-4ee1-4a9a-b2da-06d79e975403
ObjectCategory                     : ResourceGroup.co.uk/Configuration/Schema/ms-Exch-Rpc-Http-Virtual-Directory
ObjectClass                        : {top, msExchVirtualDirectory, msExchRpcHttpVirtualDirectory}
WhenChanged                        : 26/11/2013 16:23:13
WhenCreated                        : 25/11/2013 16:33:45
WhenChangedUTC                     : 26/11/2013 16:23:13
WhenCreatedUTC                     : 25/11/2013 16:33:45
OrganizationId                     :
OriginatingServer                  : thdc1.ResourceGroup.co.uk
IsValid                            : True
ObjectState                        : Changed

RunspaceId                         : aed28f13-cdd2-4dcf-a0a8-96b8e2518171
ServerName                         : THCAS2
SSLOffloading                      : True
ExternalHostname                   : thcas2.resourcegroup.co.uk
InternalHostname                   : thcas2.resourcegroup.co.uk
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Basic, Ntlm, Negotiate}
XropUrl                            :
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : False
MetabasePath                       : IIS://THCAS2.ResourceGroup.co.uk/W3SVC/1/ROOT/Rpc
Path                               : C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\rpc
ExtendedProtectionTokenChecking    : None
ExtendedProtectionFlags            : {}
ExtendedProtectionSPNList          : {}
AdminDisplayVersion                : Version 15.0 (Build 712.24)
Server                             : THCAS2
AdminDisplayName                   :
ExchangeVersion                    : 0.20 (15.0.0.0)
Name                               : Rpc (Default Web Site)
DistinguishedName                  : CN=Rpc (Default Web Site),CN=HTTP,CN=Protocols,CN=THCAS2,CN=Servers,CN=Exchange
                                     Administrative Group (FYDIBOHF23SPDLT),CN=Administrative
                                     Groups,CN=ResourceGroup,CN=Microsoft
                                     Exchange,CN=Services,CN=Configuration,DC=ResourceGroup,DC=co,DC=uk
Identity                           : THCAS2\Rpc (Default Web Site)
Guid                               : 39c5133f-14cb-4d44-ae9c-69f7fb0fc432
ObjectCategory                     : ResourceGroup.co.uk/Configuration/Schema/ms-Exch-Rpc-Http-Virtual-Directory
ObjectClass                        : {top, msExchVirtualDirectory, msExchRpcHttpVirtualDirectory}
WhenChanged                        : 26/11/2013 16:23:05
WhenCreated                        : 26/11/2013 12:49:17
WhenChangedUTC                     : 26/11/2013 16:23:05
WhenCreatedUTC                     : 26/11/2013 12:49:17
OrganizationId                     :
OriginatingServer                  : thdc1.ResourceGroup.co.uk
IsValid                            : True
ObjectState                        : Changed

If anyone could offer any help it'd be much appreciated, it's going to take several days to migrate my remaining 2010 users to 2013 which is causing a lot of use complaints.