Scenario:
I set up successfully TFS2010 webdeploy task for solution. Everything worked fine until suddendly something went wrong in the deployment task.
Solu
I encountered the same issue when building via TFS. When I tried to manually import the website I got a more informative error: "not able to log on the user \WDeployConfigWriter".
Turns out that when you install web deploy it sets up two local accounts WDeployConfigWriter and WDeployAdmin. The passwords on these accounts are set to expire. So reset the passwords on the web server and set to "never expire". Then go to Management Service Delegation in IIS. Each of the presented rules has a UserName field. Where it is WDeployAdmin or WDeployConfigWriter right click and update the credentials to the new passwords.
A full explanation with screenshots can be found here: http://workinghardinit.wordpress.com/2011/07/18/wdeployconfigwriter-account-issues-trouble-shooting-web-deploy-2-0-with-lessons-learned/