Authentication problem (web service call)

Hi Newsgroup,

I hope, that I've choosen the correct NG for my problem:

We've got a website on a server, which is calling a web service
(/SiteService/SiteService.asmx) on another webserver.
This web service-call fails and I receive the following line in the iis log:

2007-08-30 13:21:28 W3SVC1 10.1.2.3 POST /SiteService/SiteService.asmx -
8081 - 10.1.2.4
Mozilla/4.0+(compatible;+MSIE+6.0;+MS+Web+Services+Client+Pr otocol+1.1.4322.2407)
401 1 2148074252

Here are all lines, which are logged at the timestamp of the web service
call:
2007-08-30 13:21:28 W3SVC1 10.1.2.3 POST /SiteService/SiteService.asmx -
8081 - 10.1.2.4
Mozilla/4.0+(compatible;+MSIE+6.0;+MS+Web+Services+Client+Pr otocol+1.1.4322.2407)
401 2 2148074254
2007-08-30 13:21:28 W3SVC1 10.1.2.3 POST /SiteService/SiteService.asmx -
8081 - 10.1.2.4
Mozilla/4.0+(compatible;+MSIE+6.0;+MS+Web+Services+Client+Pr otocol+1.1.4322.2407)
401 1 0
2007-08-30 13:21:28 W3SVC1 10.1.2.3 POST /SiteService/SiteService.asmx -
8081 - 10.1.2.4
Mozilla/4.0+(compatible;+MSIE+6.0;+MS+Web+Services+Client+Pr otocol+1.1.4322.2407)
401 1 2148074252

I found out, that the part "401 1 2148074252" means:
401.1 --> Denied by Invalid User Credentials
2148074252 --> 'Unknown user or bad password'

The username and password, which are configured in the web site for calling
the web service are correct (I've double checked it several times).

I rebuilded this productive environment in a VPC environment:
In this environment I don't receive this error. Everything is fine:
2007-08-30 13:49:39 W3SVC1 10.1.2.3 POST /SiteService/SiteService.asmx -
8081 DOMAIN\adminsps 10.1.2.4
Mozilla/4.0+(compatible;+MSIE+6.0;+MS+Web+Services+Client+Pr otocol+1.1.4322.2407)
200 0 0

What I noticed: in this successful log entry there is the username
(DOMAIN\adminsps) included. In the unsuccessul log entry of the productive
environment the username is missing.
Could that be the reason for this error?

How can I solve or go on with this problem?

One additional note from me:
Since I had the correct SPNs in the productive environment I can call the
webservice-site manually (with the IE) and can reach the site without an
error.
Which means, that there is no kerberos error, correct?

Regards,
Katrin