When you are trying to install Dynamics AX 2009 Enterprise Portal and for Dynamics AX you don't have installed all languages the Enterprise Portal setup might fail.

In the DynamicsSetup.txt you find the following error:

Creating new Enterprise Portal Site with following the parameters:\n URL: http://server/sites/DynamicsAX\n Title: Enterprise Portal Site\n LCID: 1033\n OwnerLogin: domain\user
Administration Web Service URL: http://server:44901/_layouts/ep/WSSAdmin.asmx 
Error: The site /sites/DynamicsAX could not be created.  The following exception occured: File or arguments not valid for site template 'AXSITEDEF#0'.. 

An error occured while Setup was creating a new site. 
Exception of type 'Microsoft.SharePoint.SoapServer.SoapServerException' was thrown. 

File or arguments not valid for site template 'AXSITEDEF#0'. 
Exception of type 'Microsoft.SharePoint.SoapServer.SoapServerException' was thrown. 

The significant part of this error message is:

File or arguments not valid for site template 'AXSITEDEF#0'.

You typically run into this issue if you have in Dynamics AX only "en-gb" (not "en-us") licensed and try to create the Enterprise Portal site on a Windows SharePoint Services server that is in "en-us" installed. But the issue is not limited to "en-gb", you can experience similar issues with "de-ch" on a German Windows SharePoint Services server.

This issue is solved with the Application Hotfix KB 960048.

If after installing the Hotfix the Enterprise Portal installation is still failing with the same error, recompile the SysEpDeployment class on a PC where the Dynamics AX Client and the .NET Business Connector are both installed.