NTLM and man-in-the-middle proxies not working

While doing an application evaluation using  man-in-the-midle proxies 
(Odysseus, then Paros, Achilles) I've found an internal site that doesn't work. 
Since it seems to be independent of the proxies used and really protects the 
aplication from fuzzing with parameters, it seems appropriate to seek help here.

Normal Scenario is the following:
-IIS6 servers with IWA
-Application validates users through IWA

Internal Browser configured with man-in-the-middle proxy scenario is:
-Access to the site gives an 401 HTTP error, which also occurs above, but here 
IWA information is not sent by the browser
-No object is visible on the site
-HTTP Headers and code are the same in both circunstances
-Advanced Internet Options in IE were explored, with no result, including "Use 
HTTP/1.1 through proxy connections" and "Enable Integrated Windows 
AUthentication"
-Changing Security zones in IE were also tried, but also with no result

External Browser configured with man-in-the-middle proxy scenario is:
-Access to the application is possible, after authentication information is 
inserted
-Application denies access to the same user that works in the normal scenario
-GIF objects are visible though

Seems there's a problem with the way the browser/user/machine is being 
validated. IP validation is not an option, as testing were always done with the 
same IP. I've a fealing this might be related to Kerberos or something like GP. 

Any ideas?

rj