Recently I have seen this issue when trying to create an Enterprise Portal site with SharePoint 2010. The setup failed when creating an EP site via the AX setup and when creating a new site collection via SharePoint central admin onto a newly created web application.
The important part of the setup log was:

Entering function CurrentUserAlias
Leaving function CurrentUserAlias
Creating new Enterprise Portal Site with following the parameters:\n URL: http://axserv/sites/DynamicsAx\n Title: EP Site\n LCID: 1033\n OwnerLogin: CONTOSO\axadmin
Administration Web Service URL: http://axserv:1255/_layouts/ep/WSSAdmin.asmx
Information: Debugging mode is off.
An error occured while Setup was creating a new site.
Client found response content type of 'text/html; charset=utf-8', but expected 'text/xml'.
The request failed with the error message:

Within the setup log, after the section copied above, there was some HTML code, we copied this out into notepad and saved as html to get a better look at it, what it showed was: 

Error
An unexpected error has occurred.
Troubleshoot issues with Microsoft SharePoint Foundation.
Correlation ID: 43d5fd87-a6b3-472b-92bc-de3c5b16066f
Date and Time: 7/22/2011 5:24:06 PM
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We're using the standard WSS admin web service to create the EP site, we just call the web service and send out XML that creates all our pages and Web Parts etc.
In this case it appears that it failed in the first call to the web service.

Solution:
The EP site creation failed because the correct permissions were not setup within Central Admin so therefore the Web Service could not be called.
The solution was to add the Business Connector Service account to the Central Admin Farm Administrators Group, then the EP site could be created without throwing any errors.

--author:  Charlotte Rowe
--editor: Tariq Bell
--date: 15/08/2011