Cross Domain Data Source Error in SSRS 2008

Question

We have our data warehouse server on a different Windows domain (Domain ABC) to the domain where our end user tools such as SSRS and SSAS sit (DOMAIN 123).  Historically we've used SQL server mixed mode authentication to get around any cross domain
authentication issues but to tighten up security we are trying to switch to Windows only.  However, I'm unable to my development instance of SSRS to connect to a datasource using credentials from a different domain.
I created a copy of the live data source with the same settings except for the credentials.  Here I selected Windows credentials (without impersonation ticked) and put in the username with the format DOMAINABC\DomainAccount with a domain account from
the same domain as our DWH server.  However, none of the reports I tested can conect and throw the error below.  Please can anyone give me some pointers on things to check?  My google research has drawn a blank so far!
An error occurred during client rendering.
An error has occurred during report processing.
Cannot impersonate user for data source 'DWH'.
Log on failed.
Logon failure: unknown user name or bad password. (Exception from HRESULT: 0x8007052E) 

Answer 1

From your inital post, it sounded like you wanted to login to the data source with a remote domain user (123 in your example). Glad to see that's not the case!
When you tested the credentials with SSMS were you on the remote domain?
It looks to me like the impersonation is implicit: https://msdn.microsoft.com/en-us/library/ms160330.aspx
Theres a couple of notes at the top of that page about remote access being enabled, and the ports that should be open too.
Theres a bunch of potentially useful information about what your connection string should look like here too:
https://msdn.microsoft.com/en-us/library/ms156450.aspx