AbleCommerce Shopping Cart Software

Hi everyone. I need some help with this issue.
My System is as follow:
Cart: AC7.0.3 Net Framework v 3.5
Web server: Virtual Machine Windows Server 2008 R2 SP1
Data base: Remote 2 node Cluster SQL Server 2008 R2 SP1 for HA (same domain as Web server)
Authentication: Integrated Windows (Domain Admin)
Root, Admin, App_Data, and Install folders owned by the Domain admin with full control
IIS_IUSRS full control granted
for the database connection I choose:
Specify database=>Server Name: Server\instance name
Database Name=> created empty Db named AbleCommerce
Database User=>domain\username
SQL cluster and SQL Servers added to the Web server administrator group
Testing an ODBC connection....
Microsoft SQL Server Native Client Version 10.50.2500
Running connectivity tests...
Attempting connection
Connection established
Verifying option settings
Disconnecting from server
TESTS COMPLETED SUCCESSFULLY!
No matter how many times and different connections variations I try, the result are always the same:
The specified database connection could not be opened. followed by the generated connection string
Obviously something is misconfigured and I'm missing it!
Help please
Thank you

Does SQL server allow remote connections? That's all I can think of.

Thank you Judy for take you time to trying to help me with this.
Yes, the server allow remote connections, on WS2K8R2 it is activated by default with a timeout query to it of 600 sec.
I been putting some 60 hrs. or so doing troubleshooting, installed and configured a local WS2K8R2, a local WS2K5Expess alternatively pointing theAC7 install to the correspondent one, create a new user as dbo, ran portquery and hping on tcp 1433, I rdp from the webserver to the remote cluster with no problem.
What else could be authentication problem?
Keeping on digging
Thank you

Problem Solved!!
even better being able to migrated the DB from original to target with no errors
piece of advice to the no seasoned like me: read the instructions through and through, once and again!, little subtleties you took for granted 'boomerang around' to hit your unaware rear end.
do a:
1. mixed authentication with sa as dbo owner
2. on ssms check management->SQl Server Logs will inform you (live) what the errors are, then baby walked to what to do next, just an overlooked classic tool!
3. follow the toolbox troubleshooting techniques of your arsenal and you are done
Merry Christmas to all (check the 2010 Columbia Valley Cabernet Sauvignon, wash anything eaten down pretty good